[PATCH -next] ARM: OMAP2+: Fix build warning when mmc_omap is not built

[Tony Lindgren]

* Yongqiang Liu <liuyongqiang13 at huawei.com> [210401 16:09]:
> 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().

Thanks applying into fixes.

Tony