xfrm: Fix installation of AH IPsec SAs

The SPI check introduced in ea9884b3
was intended for IPComp SAs but actually prevented AH SAs from getting
installed (depending on the SPI).

Fixes: ea9884b3

 ("xfrm: check user specified spi for IPComp")
Cc: Fan Du <fan.du@windriver.com>
Signed-off-by: Tobias Brunner <tobias@strongswan.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>