mmc: Fix bug in sd_set_card_speed()

After settings the speed of the sd with the switch command, a check is
done to make sure that the new speed has been set. The current check has a
masking error: speed are encoded on 4 bits only.
Fix it by masking the upper bits.

This fixes a problem seen with QEmu emulating a vexpress-a15.

Reported-by: Jonathan Gray <jsg@jsg.id.au>
Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Tested-by: Jonathan Gray <jsg@jsg.id.au>