serial/amba-pl011: Disable interrupts around TX softirq

serial/amba-pl011: Disable interrupts around TX softirq

pl011_tx_softirq() currently uses spin_{,un}lock(), which are not
sufficient to inhibit pl011_int() from being triggered by a local
IRQ and trying to re-take the same lock.  This can lead to
deadlocks.

This patch uses the _irq() locking variants instead to ensure that
pl011_int() handling for a given port is deferred until any
pl011_tx_softirq() work for that port is complete.

Notes for stable:

This patch fixes an issue that is fixed by the following upstream
commit, which is a more substantial rewrite of the affected code,
fixing multiple, mostly more minor issues:

        1e84d22322ceed4767db1e5342c830dd60c8210f
        serial/amba-pl011: Refactor and simplify TX FIFO handling

The upstream patch was rejected for stable on the reasonable grounds
that it was too big and complex a patch.  The original buggy code was
merged in v4.1, and the rewrite was merged in v4.2, leaving only v4.1
affected.

This patch replaces the 1e84d22, for 4.1.x only.

Fixes: 734745caeb9f serial/amba-pl011: Activate TX IRQ passively
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Tested-by: Robin Murphy <robin.murphy@arm.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>