drivers/mmc/mmc.c: Fix build warning

Fix:
mmc.c: In function 'mmc_bounce_buffer_start':
mmc.c:132:13: warning: no return statement in function returning
non-void [-Wreturn-type]

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Andy Fleming <afleming@gmail.com>
Cc: Marek Vasut <marex@denx.de>
Acked-by: Marek Vasut <marex@denx.de>