smsc: Remove phy address check in smsc_miibus_writereg

[emaste]

Summary

The driver only allowed writes to the expected phy (i.e., sc->sc_phyno),
and reads were never filtered.  sc_phyno is already passed to
mii_attach() as phyloc, so nothing tries to issue MDIO writes to other
addresses and the check is not required.

Remove the check in advance of supporting external PHYs at arbitrary
addresses.