iflib: netmap: Declare the correct number of rx descriptor

[manu]

Summary

When a driver is declaring supporting IFLIB_HAS_RXCQ, we do not use the
free list 0 but the free list 1, which may not contain the same number of
available descriptors.
This fixes using netmap on the vmxnet3 driver (and probably on the if_bnxt one
too).
MFC-After: 1 week
Sponsored by: EfficientIP

Test Plan

Tested on VMWare esxi on 12-stable and 13-current

[manu]

Not needed anymore