wifi: mt76: mt7915: Fix an error handling path in mt7915_add_interface()

wifi: mt76: mt7915: Fix an error handling path in mt7915_add_interface()

If mt76_wcid_alloc() fails, the "mt76.mutex" mutex needs to be released as
done in the other error handling paths of mt7915_add_interface().

Fixes: f3049b88b2b3 ("wifi: mt76: mt7915: allocate vif wcid in the same range as stations")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://patch.msgid.link/b9d8fbfc19360bfe60b9cea1cb0f735ab3b4bc26.1727639596.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Felix Fietkau <nbd@nbd.name>