tg3: Clarify semantics of TG3_IRQ_MAX_VECS

TG3_IRQ_MAX_VECS should be seen as the maximum number of vectors that
any device could be expected to use.  tp->irq_max represents the maximum
number of vectors the current device can use.  This patch clarifies the
semantics of the code to match the above description.

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