powerpc/tau: Convert from timer to workqueue

powerpc/tau: Convert from timer to workqueue

Since commit 19dbdcb8039cf ("smp: Warn on function calls from softirq
context") the Thermal Assist Unit driver causes a warning like the
following when CONFIG_SMP is enabled.

  ------------[ cut here ]------------
  WARNING: CPU: 0 PID: 0 at kernel/smp.c:428 smp_call_function_many_cond+0xf4/0x38c
  Modules linked in:
  CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.7.0-pmac #3
  NIP:  c00b37a8 LR: c00b3abc CTR: c001218c
  REGS: c0799c60 TRAP: 0700   Not tainted  (5.7.0-pmac)
  MSR:  00029032 <EE,ME,IR,DR,RI>  CR: 42000224  XER: 00000000
  GPR00: c00b3abc c0799d18 c076e300 c079ef5c c0011fec 00000000 00000000 00000000
  GPR08: 00000100 00000100 00008000 ffffffff 42000224 00000000 c079d040 c079d044
  GPR16: 00000001 00000000 00000004 c0799da0 c079f054 c07a0000 c07a0000 00000000
  GPR24: c0011fec 00000000 c079ef5c c079ef5c 00000000 00000000 00000000 00000000
  NIP [c00b37a8] smp_call_function_many_cond+0xf4/0x38c
  LR [c00b3abc] on_each_cpu+0x38/0x68
  Call Trace:
  [c0799d18] [ffffffff] 0xffffffff (unreliable)
  [c0799d68] [c00b3abc] on_each_cpu+0x38/0x68
  [c0799d88] [c0096704] call_timer_fn.isra.26+0x20/0x7c
  [c0799d98] [c0096b40] run_timer_softirq+0x1d4/0x3fc
  [c0799df8] [c05b4368] __do_softirq+0x118/0x240
  [c0799e58] [c0039c44] irq_exit+0xc4/0xcc
  [c0799e68] [c000ade8] timer_interrupt+0x1b0/0x230
  [c0799ea8] [c0013520] ret_from_except+0x0/0x14
  --- interrupt: 901 at arch_cpu_idle+0x24/0x6c
      LR = arch_cpu_idle+0x24/0x6c
  [c0799f70] [00000001] 0x1 (unreliable)
  [c0799f80] [c0060990] do_idle+0xd8/0x17c
  [c0799fa0] [c0060ba8] cpu_startup_entry+0x24/0x28
  [c0799fb0] [c072d220] start_kernel+0x434/0x44c
  [c0799ff0] [00003860] 0x3860
  Instruction dump:
  8129f204 2f890000 40beff98 3d20c07a 8929eec4 2f890000 40beff88 0fe00000
  81220000 552805de 550802ef 4182ff84 <0fe00000> 3860ffff 7f65db78 7f44d378
  ---[ end trace 34a886e47819c2eb ]---

Don't call on_each_cpu() from a timer callback, call it from a worker
thread instead.

Fixes: 1da177e4c3f41 ("Linux-2.6.12-rc2")
Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/bb61650bea4f4c91fb8e24b9a6f130a1438651a7.1599260540.git.fthain@telegraphics.com.au