ionic: use pci_is_enabled not open code

Since there is a utility available for this, use
the API rather than open code.

Fixes: 13943d6c

 ("ionic: prevent pci disable of already disabled device")
Reviewed-by: Brett Creeley <brett.creeley@amd.com>
Signed-off-by: Shannon Nelson <shannon.nelson@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>