bnxt_en: Update VNIC resource calculation for VFs

Newer versions of firmware will pre-reserve 1 VNIC for every possible
PF and VF function.  Update the driver logic to take this into account
when assigning VNICs to the VFs.  These pre-reserved VNICs for the
inactive VFs should be subtracted from the global pool before
assigning them to the active VFs.

Not doing so may cause discrepancies that ultimately may cause some VFs to
have insufficient VNICs to support features such as aRFS.

Signed-off-by: Vikas Gupta <vikas.gupta@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>