OMAP: cs should be positive in gpmc_cs_free()

The index `cs' is signed, test whether it is negative before we release
gpmc_cs_mem[cs].

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>