PASN: Fix Authentication frame checks

PASN: Fix Authentication frame checks

The way type and subtype of the FC field was checked does not really
work correctly. Fix those to check all bits of the subfields. This does
not really make any practical difference, though, since the caller was
already checking this.

Furthermore, use a helper function to avoid having to maintain two
copies of this same functionality.

Signed-off-by: Jouni Malinen <j@w1.fi>