tg3: Fix missed MSI workaround

This patch fixes a minor counter initialization bug and makes the MSI
workaround slightly more efficient by attempting to service pending
interrupts before applying the workaround.

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