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