xfrm: Allow transport-mode states with AF_UNSPEC selector

xfrm state selectors are matched against the inner-most flow
which can be of any address family.  Therefore middle states
in nested configurations need to carry a wildcard selector in
order to work at all.

However, this is currently forbidden for transport-mode states.

Fix this by removing the unnecessary check.

Fixes: 13996378 ("[IPSEC]: Rename mode to outer_mode and add inner_mode")
Reported-by: David George <David.George@sophos.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>