ix(4): Report RX errors as sum of all RX error counters

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 <piotr.pietruszewski@intel.com>

Reviewed By: erj
Tested By: gowtham.kumar.ks@intel.com
Sponsored By: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D27191

(cherry picked from commit afb1aa4e6df245d38fd2ba683fa521d5dabe8392)