HW keeps track of RX errors using several counters, each for
specific type of errors. Report RX errors to OS as sum
of all those counters: CRC errors, illegal bytes, checksum,
length, undersize, fragment, oversize and jabber errors.
Also, add new "rx_errs" sysctl in the dev.ix.N.mac_stats tree. This is
to provide an another way to display the sum of RX errors.
Signed-off-by: Piotr Pietruszewski <email@example.com>