serial: amba-pl011: fix regression, causing an Oops on rmmod

serial: amba-pl011: fix regression, causing an Oops on rmmod

A recent commit e6bc87a8a62b20b29013c91fbaaad7cbdd87bed5 "serial: pl011:
Move uart_register_driver call to device probe" introduced a regression,
causing the pl011 driver to Oops if more than 1 port have been probed. Fix
the Oops by only calling uart_unregister_driver() once after the last port
has been removed.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>