Fix CID 102619 in the Coverity report on Linux.

efx_end_loopback() iterates over an array of skb pointers of which
some may be null (if efx_begin_loopback() failed).  It should not use
dev_kfree_skb_irq(), which requires non-null pointers.  In practice
this is safe because it does not run in interrupt context and
therefore always ends up calling dev_kfree_skb(), which does allow
null pointers.  But we should make that explicit.
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>