bnxt_en: Do not include ETH_FCS_LEN in the max packet length sent to fw.

The firmware does not expect the CRC to be included in the length
passed from the driver.  The firmware always configures the chip
to strip out the CRC.

Signed-off-by: Vasundhara Volam <vasundhara-v.volam@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>