During attach we release the lock to call e6000sw_attach_miibus(), which
calls mii_attach(). The mii_attach() function calls miibus_readreg() /
miibus_writereg(). However, these are set to be
e6000sw_readphy_locked().
That is, the read/write phy functions do not acquire the lock, but
expect to be called while locked. Simply do not unlock and relock while
calling mii_attach().
Sponsored by: Rubicon Communications, LLC ("Netgate")