MFC r325840, r327811, and r329701.

r325840:
CXGBE: fix big-endian behaviour

The setbit/clearbit pair casts the bitfield pointer
to uint8_t* which effectively treats its contents as
little-endian variable. The ffs() function accepts int as
the parameter, which is big-endian. Use uint8_t here to
avoid mismatch, as we have only 4 doorbells.

Submitted by: Wojciech Macek <wma@freebsd.org>
Reviewed by: np
Obtained from: Semihalf
Sponsored by: QCM Technologies
Differential revision: https://reviews.freebsd.org/D13084

r327811:
CXGBE: fix get_filt to be endianness-aware

Unconditional 32-bit shift is not endianness-safe.
Modify the logic to work both on LE and BE.

Submitted by: Wojciech Macek <wma@freebsd.org>
Reviewed by: np
Obtained from: Semihalf
Sponsored by: IBM, QCM Technologies
Differential revision: https://reviews.freebsd.org/D13102

r329701:
CXGBE: implement prefetch on non-Intel architectures

Submitted by: Michal Stanek <mst@semihalf.com>
Obtained from: Semihalf
Reviewed by: np, pdk@semihalf.com
Sponsored by: IBM, QCM Technologies
Differential revision: https://reviews.freebsd.org/D14452