Correct the comments in am7990_intr() and am79900_intr(); it's not
possible to end up in the interrupt handler again while processing the
previous RX interrupt in ifp->if_input() because the MD interrupt code
disables the delivery of the respective interrupt until all associated
handlers were called (in the INTR_FILTER case the MI code supposedly
does the same). Toggling the NIC interrupt enable bit in these handlers
still is necessary though as some chips (f.e. the VMware emulated one)
require this to be done in order to keep issuing interrupts.

MFC after: 1 month