mdio_bus: Fix init if CONFIG_RESET_CONTROLLER=n

Commit 1d463956 ("mdio_bus: Fix PTR_ERR applied after initialization
to constant") accidentally changed a check from -ENOTSUPP to -ENOSYS,
causing failures if reset controller support is not enabled.  E.g. on
r7s72100/rskrza1:

    sh-eth e8203000.ethernet: MDIO init failed: -524
    sh-eth: probe of e8203000.ethernet failed with error -524

Seen on r8a7740/armadillo, r7s72100/rskrza1, and r7s9210/rza2mevb.

Fixes: 1d463956 ("mdio_bus: Fix PTR_ERR applied after initialization to constant")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: YueHaibing <yuehaibing@huawei.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>