bnxt_en: Fix "uninitialized variable" bug in TPA code path.

In the TPA GRO code path, initialize the tcp_opt_len variable to 0 so
that it will be correct for packets without TCP timestamps.  The bug
caused the SKB fields to be incorrectly set up for packets without
TCP timestamps, leading to these packets being rejected by the stack.

Reported-by: Andy Gospodarek <andrew.gospodarek@broadocm.com>
Acked-by: Andy Gospodarek <andrew.gospodarek@broadocm.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>