[SERIAL] Fix status reporting with PL011 serial driver

[SERIAL] Fix status reporting with PL011 serial driver

The receiver status register reports latched error conditions, which
must be cleared by writing to it.  However, the data register reports
unlatched conditions which are associated with the current character.
Use the data register to interpret error status rather than the RSR.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>