net: qrtr: Allow control port access to AID_VENDOR_QRTR

Only allow processes to bind to the control port if they are in the
AID_VENDOR_QRTR group or have NET_ADMIN capabilities. This GID is
reserved for QRTR control applications only.

Remove NET_BIND_SERVICE check from netlink socket callback. This is a
redundant check since NET_ADMIN is a more priveleged capability.

Change-Id: I70961418314c391017709cab6408e5c58cf6d848
Signed-off-by: Chris Lew <clew@codeaurora.org>