IB/srpt: Drop pointless static qualifier in srpt_make_tpg()

There is no need to have the 'struct se_portal_group *tpg' variable static
since new value always be assigned before use.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>