xilinx: common: fix board_late_init_xilinx()

xilinx: common: fix board_late_init_xilinx()

Compiling with GCC-12 leads to an error:

    +board/xilinx/common/board.c:479:37: error: the comparison will always
     evaluate as 'true' for the address of 'mac_addr' will never be NULL
     [-Werror=address]
    +  479 |                                 if (!desc->mac_addr[i])
    +      |                                     ^

Remove the redundant check.

Fixes: c38f8245711b ("xilinx: board: Add support for additional card detection")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Link: https://lore.kernel.org/r/20221008091317.52838-1-heinrich.schuchardt@canonical.com
Signed-off-by: Michal Simek <michal.simek@amd.com>