miivar.h includes opt_platform.h. Make sure all the drivers that use the
miibus_if.h interface file have opt_platform.h as well. While some of
these may not, strictly speaking, need it, it's easier to include it
universally for miibus.
Sponsored by: Netflix
Fix detach routine.
Driver was tested on EspressoBin.
Remove it from GENERIC, since now it can be loaded automatically.
Obtained from: Semihalf
Reviewed by: manu
Differential revision: https://reviews.freebsd.org/D31580