mt76: usb: remove WARN_ON in mt76u_get_rx_entry_len

Remove not useful WARN_ON macros in mt76u_get_rx_entry_len routine
since corrupted frames should just be silently discarded

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