wifi: mt76: add a wrapper for wcid access with validation

wifi: mt76: add a wrapper for wcid access with validation

Several places use rcu_dereference to get a wcid entry without validating
if the index exceeds the array boundary. Fix this by using a helper function,
which handles validation.

Link: https://patch.msgid.link/20250707154702.1726-1-nbd@nbd.name
Signed-off-by: Felix Fietkau <nbd@nbd.name>