net/bnxt: fix doorbell register offset for Tx ring

For Tx-ring # 104 and higher, the doorbell register was incorrectly
configured due to which FW was not able to receive the notification
of packet to transmit.
With this fix, user can run traffic upto 256 rings.

Fixes: 6eb3cc2294fd ("net/bnxt: add initial Tx code")
Cc: stable@dpdk.org
Signed-off-by: Rahul Gupta <rahul.gupta@broadcom.com>
Signed-off-by: Ajit Khaparde <ajit.khaparde@broadcom.com>