liquidio: disallow enabling firmware debug from a VF

Disallow enabling firmware debug from a VF.  Only PF is allowed to do that.

Signed-off-by: Derek Chickles <derek.chickles@cavium.com>
Signed-off-by: Felix Manlunas <felix.manlunas@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>