spl: mmc: Fix spl_mmc_get_uboot_raw_sector() implementation

The call to spl_mmc_get_uboot_raw_sector() completely ignores and
overwrites the raw_sect value passed from the caller of spl_mmc_load().

Fix this by passing raw_sect to the function and returning the same
value in the default case.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>