cc: Sasha Hauer <kernel@pengutronix.de>

We noticed that we were only able to communicate once with a GSM module
per session, if we powered down the module then the serial driver was
broken until reset, no traffic or /proc/interrupts activity could be
seen any more.

Volker noticed it was provoked by a long "break" seen on the wire when
the RX was not driven during powerdown, we discovered we could kill the
serial driver just by forcing RX low extenally for a little while.

I saw that the processing for BREAK in the driver gets too excited and
clears down all set bits in USR2, including that a character is pending.
This stops all further characters getting processed.

The attached one-liner makes the serial driver immortal against BREAK.
Signed-off-by: NAndy Green <andy.green@txtr.com>
Signed-off-by: NVolker Ernst <volker.ernst@txtr.com>
Signed-off-by: NSascha Hauer <s.hauer@pengutronix.de>