i2c-pxa2xx: check timeout correctly

timeout here maybe 0 if the event occured and a task with a higher
priority stole the cpu and we were sleeping longer than the timeout
value we specified.
In case of a real timeout I changed the error code to I2C_RETRY so we
retry the transfer.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Dirk Brandewie <dirk.brandewie@gmail.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>