spl: mmc: use block device number, not hard-coded 0

In order to support boot from multiple devices through board_boot_order,
it's necessary to use the block number of a device.

The use of a hard-coded 0 for the device number also creates a need
to re-order block devices for use in SPL like this:
	http://git.denx.de/?p=u-boot.git;a=blob;f=board/freescale/mx6slevk/mx6slevk.c;hb=HEAD#l195



Signed-off-by: Eric Nelson <eric@nelint.com>