Fix compiler warnings in gpmc.c

Fix these compiler warnings:

gpmc.c: In function 'gpmc_init':
gpmc.c:432: warning: 'return' with a value, in function returning void
gpmc.c:439: warning: 'return' with a value, in function returning void

Signed-off-by: Sanjeev Premi <premi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>