i40evf: detect reset more reliably

Using VFGEN_RSTAT to detect a VF reset is an endeavor that is fraught
with peril. It's entirely too easy to miss a reset because none of the
bits are sticky. By the time the VF driver reads the register, the reset
may have been processed and cleaned up by the PF driver, leaving the
register in the same state that it was before the reset.

Instead, detect a reset with the VF_ARQLEN register. When the VF is
reset, the enable bit in this register is cleared, and it stays cleared
until the VF driver processes the reset and re-enables the admin queue.

Because we now deal with multiple registers in the reset and watchdog
tasks, rename the rstat_val variable to reg_val.

Change-ID: Id1df17045c0992e607da0162d31807f7fc20d199
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>