ems_usb: cleanup: remove uneeded check

"skb" is alway non-null here, but even if it were null the check isn't
needed because dev_kfree_skb() can handle it.

This eliminates a smatch warning about dereferencing a variable before
checking that it is non-null.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>