OMAP3/4/5/AM33xx: Correct logic for checking FAT or RAW MMC

In the case of booting from certain peripherals, such as UART, we must
not see what the device descriptor says for RAW or FAT mode because in
addition to being nonsensical, it leads to a hang.  This is why we have
a test currently for the boot mode being within range.  The problem
however is that on some platforms we get MMC2_2 as the boot mode and not
the defined value for MMC2, and in others we get the value for MMC2_2.
This is required to fix eMMC booting on omap5_uevm.

Tested on am335x_evm (UART, NAND, SD), omap3_beagle (NAND, SD on
classic, SD only on xM rev C5) and omap5_uevm (SD, eMMC).

Signed-off-by: Tom Rini <trini@ti.com>