s390/qeth: Fix vnicc_is_in_use if rx_bcast not set

Symptom: After vnicc/rx_bcast has been manually set to 0,
	bridge_* sysfs parameters can still be set or written.
Only occurs on HiperSockets, as OSA doesn't support changing rx_bcast.

Vnic characteristics and bridgeport settings are mutually exclusive.
rx_bcast defaults to 1, so manually setting it to 0 should disable
bridge_* parameters.

Instead it makes sense here to check the supported mask. If the card
does not support vnicc at all, bridge commands are always allowed.

Fixes: caa1f0b1 ("s390/qeth: add VNICC enable/disable support")
Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>