xfrm: delete duplicated functions that calls same xfrm_api_check()

The xfrm_dev_register() and xfrm_dev_feat_change() have same
implementation of one call to xfrm_api_check(). Instead of doing such
indirection, call to xfrm_api_check() directly and delete duplicated
functions.

Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>