i2c-pxa: Misc fixes

While working on the PCA9564-platform driver, I sometimes had a glimpse at the
pxa-driver. I found some suspicious places, and this patch contains my
suggestions. Note: They are not tested, due to no hardware.

[JD: Some more fixes.]

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Tested-by: Mike Rapoport <mike@compulab.co.il>
Tested-by: Eric Miao <ymiao3@marvell.com>