if: Add subqueue watchdog helper functions for driver use

Ease driver's multiple TX queue support
igb: Utilize subqueue watchdog functions

Prepare for the multiple TX queue support
igb: Setup subqueue CPUID before possible calling of igb_init
igb: Use ifsq_devstart_sched in watchdog and resume methods

So these two method won't consume too much time.

In watchdog method, schedule all subqueues instead of the subqueue
causing watchdog timeout.
igb: Don't assume only one TX queue will be enabled.

Prepare multiple TX queue support.

- Add TX queue count when MSI-X is used
- Add currently used TX queue count

Number of TX queues are still limited to 1 as of this commit.