target: fixup error message in target_tg_pt_gp_alua_access_type_store()

When setting up a target the error message:

Unable to do set ##_name ALUA state on non valid tg_pt_gp ID: 0

is displayed.
Apparently concatenation doesn't work in a string; one should be using
implicit string concatenation here.

Signed-off-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Bart van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>