mvneta: Fix 64-bit MIB reads

It appears we must read MIB values as 2 4-byte words, lower address
first. A single 8-byte MIB read returns the value with the lower 4
bytes copied into the upper 4 bytes, resulting in bogus byte counter
values.

Reviewed by: mw
MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC (Netgate)
Differential Revision: https://reviews.freebsd.org/D27870