ifcapnv: fix IFCAP2 usage

IFCAP2_XXX constants are integers, they do not need shift for the
definition. But their usage as bitmask for if_capenable2 does require
shift. Add convenience macro IFCAP2_BIT() for consumers.

Fix the only existing consumer, mlx5(4) RXTLS enable bits.

Reported by: jhb
Reviewed by: jhb, jhibbits, hselasky
Coverity CID: 1501659
Sponsored by: NVIDIA networking
Differential revision: https://reviews.freebsd.org/D37862