xilinx: Define only mmc devnum not partition

The commit 53b40636 ("DFU: Check the number of arguments and argument
string strictly") added strict control over string that 0:1 partition
definition is not valid anymore that's why use only device number without
partition ID. Device is specified by 2nd parameter and partition by 3rd.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/840eca944f4f2abeeb63b5d724f9ba5fe9a9213b.1660055571.git.michal.simek@amd.com