pca953x: minor include cleanup

linux/i2c/pca953x.h is a very bare include file.  Fix check for multiple
includes of linux/i2c/pca953x.h, and add dependent includes into the
header file.

Signed-off-by: Olof Johansson <olof@lixom.net>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Jean Delvare <khali@linux-fr.org>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>