mt76: mt7915: always check return value from mt7915_mcu_alloc_wtbl_req

As done for mt76_connac_mcu_alloc_wtbl_req, even if this is not a real
bug since mt7915_mcu_alloc_wtbl_req routine can fails just if nskb is NULL,
always check return value from mt7915_mcu_alloc_wtbl_req in order to avoid
possible future mistake.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>