usb: gadget: prevent potenial null pointer dereference on skb->len

An earlier fix partially fixed the null pointer dereference on skb->len
by moving the assignment of len after the check on skb being non-null,
however it failed to remove the erroneous dereference when assigning len.
Correctly fix this by removing the initialisation of len as was
originally intended.

Fixes: 70237dc8 ("usb: gadget: function: f_eem: socket buffer may be NULL")
Acked-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>