drivers: net: fsl_ls_mdio: prevent a NULL pointer dereference

Prevent a NULL pointer dereference in the probe path by checking the
return valud of dev_read_addr_ptr() against NULL.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>