tg3: Qualify use of tp->pcix_cap

This patch makes sure the device is a PCIX device before attempting to
use the pcix_cap device structure member.  This is prep work for the
following patch.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>