ARM: OMAP2+: Fix build warning when mmc_omap is not built

GCC reports the following warning with W=1:

arch/arm/mach-omap2/board-n8x0.c:325:19: warning:
variable 'index' set but not used [-Wunused-but-set-variable]
325 |  int bit, *openp, index;
    |                   ^~~~~

Fix this by moving CONFIG_MMC_OMAP to cover the rest codes
in the n8x0_mmc_callback().

Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>