NAND: TI: fix warnings in omap_gpmc.c

The following warnings are reported for boards using SOFT ECC.

omap_gpmc.c:33:30: warning: 'hw_nand_oob' defined but not used
omap_gpmc.c:78:13: warning: 'omap_hwecc_init' defined but not used
omap_gpmc.c:116:12: warning: 'omap_correct_data' defined but not used
omap_gpmc.c:182:12: warning: 'omap_calculate_ecc' defined but not used
omap_gpmc.c:208:13: warning: 'omap_enable_hwecc' defined but not used

Signed-off-by: Stefano Babic <sbabic@denx.de>
Cc: Tom Rini <trini@ti.com>
Cc: Scott Wood <scottwood@freescale.com>