Revert "usbnet: drop unneeded check for NULL"

This reverts commit 5d65878d7031b6c39054b282faceff406bb2fda9.
The upper layers call usbnet_start_xmit() with a valid skb.
However cdc_ncm abuses this method by calling it with NULL
to trigger IO for the aggregated private skb holding erlier
packets. Until cdc_ncm is fixed, the check for NULL must
be reintroduced.

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>