[ARM] 4417/1: Serial: Fix AMBA drivers locking

The -rt patch triggered a lockdep warning in the amba serial drivers, which never
shows up on UP kernels. On SMP systems this would trigger as well.

Release the port lock before calling tty_flip_buffer_push() and reacquire it after
the call. This matches the code in the 8250 serial driver.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>