The chip selects are numbered 0..(max chip selects - 1). Chip select
with number <max chip selects> is invalid. Fix the check for that. Using
the out of bound chip select may hang your board.
Signed-off-by: Stefan Lengfeld <s.lengfeld@phytec.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>