mt76: use skb_pad() instead of __skb_pad()

mt76 uses __skb_pad() with free_on_error set to true, this is the same
as calling skb_pad().
This patch does not change any functionality, but it makes it easier to
backport this driver in backports, because skb_pad() is also available
in older kernel versions.

Fixes: b40b15e1

 ("mt76: add usb support to mt76 layer")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Felix Fietkau <nbd@nbd.name>