bnxt: set hardware checksum only if required

The test condition in the bnxt driver for TCP/UDP transmit hardware checksum
offload is invalid: only the TCP / UDP csum bits should be tested

Only the relevant ipi_csum_flags bits are now tested

Reviewed by: tuexen
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D53941