ibmvscsis: Use tpgt passed in by user

ibmvscsis always returned 0 for the tpg/tag, since it did not
parse the value passed in by the user.

When functions like ALUA members exports the value, it will
be incorrect because targetcli/rtslib starts the tpg numbering
at 1.

Signed-off-by: Bryant G. Ly <bryantly@linux.vnet.ibm.com>
Signed-off-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>