cmd_mmc: make curr_device static

The curr_device variable isn't used outside of cmd_mmc, so mark it static
to avoid conflicts with other pieces of code (like sata which also exports
a curr_device).  Otherwise we end up with stuff like:
common/libcommon.a(cmd_sata.o):(.data.curr_device+0x0):
	multiple definition of `curr_device'
common/libcommon.a(cmd_mmc.o):(.data.curr_device+0x0): first defined here

Signed-off-by: Mike Frysinger <vapier@gentoo.org>