ixgbe: Do not count L3/L4 checksum errors as input errors

NIC input errors have traditionally indicated problems at the link
level (crc errors, runts, etc). People tend to build monitoring
infrastructure around such errors in order to monitor for bad network
hardware. When L3/L4 checksum errors are included in the category of
input errors, it breaks such monitoring, as these errors can originate
anywhere on the internet, and do not necessarily indicate faulty
local network hardware.

Reviewed by: erj, glebius
Differential Revision: https://reviews.freebsd.org/D38346
Sponsored by: Netflix