net/bnxt: fix Rx checksum flags

For frames where the hardware is not able to calculate checksum
we are indicating such frames to be bad. And that is incorrect.
Indicate PKT_RX_IP_CKSUM_UNKNOWN or PKT_RX_L4_CKSUM_UNKNOWN
for such frames.

Fixes: 7ec39d8c524b ("net/bnxt: update status of Rx IP/L4 CKSUM")
Cc: stable@dpdk.org
Signed-off-by: Ajit Khaparde <ajit.khaparde@broadcom.com>