net: axienet: fix a signedness bug in probe

The "lp->phy_mode" is an enum but in this context GCC treats it as an
unsigned int so the error handling is never triggered.

Fixes: ee06b172 ("net: axienet: add support for standard phy-mode binding")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>