mmc: add function prototype for mmc_set_dev in mmc.h

this eliminates compiler warnings when cmd_mmc.c is built with CONFIG_SYS_MMC_SET_DEV

the mmc_set_dev implementation in omap3_mmc.c is also tweaked to match
the new prototype in parameter naming and type

Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>