lwtunnel: remove unused but set variable

silences the below warning:
net/core/lwtunnel.c: In function ‘lwtunnel_valid_encap_type_attr’:
net/core/lwtunnel.c:165:17: warning: variable ‘nla’ set but not used
[-Wunused-but-set-variable]

Fixes: 9ed59592

 ("lwtunnel: fix autoload of lwt modules")
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>