serial: fsl_lpuart: don't restore interrupt state in ISR

serial: fsl_lpuart: don't restore interrupt state in ISR

>From commit 75f4e830fa9c ("serial: do not restore interrupt state in
sysrq helper"):

  Since commit 81e2073c175b ("genirq: Disable interrupts for force
  threaded handlers") interrupt handlers that are not explicitly
  requested as threaded are always called with interrupts disabled and
  there is no need to save the interrupt state when taking the port
  lock.

Apply this also to fsl_lpuart in prepartion for sysrq handling with
uart_unlock_and_check_sysrq().

Signed-off-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20210512141255.18277-4-michael@walle.cc
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>