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