mt76: usb: do not build the skb if reported len does not fit in buf_size

Precompute data length in order to avoid to allocate the related
skb data structure if reported length does not fit in queue buf_size

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>