genet: fix problems with interface down/up

The genet interface did not resume operation correctly after doing
ifconfig down then up. The down/reset procedure did not clear the
RUNNING flag, and did not reset enough of the hardware state. This
patch is modeled on OpenBSD code, with a call to gen_reset added
to reset the controller completely. Regularize the parameter to
gen_dma_disable() while here.

PR: 263091
Submitted by: jiahali@blackberry.com