net: lan966x: fix a IS_ERR() vs NULL check in lan966x_create_targets()

The devm_ioremap() function does not return error pointers.  It returns
NULL.

Fixes: db8bcaad ("net: lan966x: add the basic lan966x driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>