bnx2: Remove timer_interval field from the bnx2 structure

The timer_interval field is only assigned once, and never reassigned.
We can safely replace all instances of the timer_interval with a
constant value.

Signed-off-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Acked-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: David S. Miller <davem@davemloft.net>