IB/srpt: Ensure that modifying the use_srq configfs attribute works

The use_srq configfs attribute is created after it is read. Hence
modify srpt_tpg_attrib_use_srq_store() such that this function
switches dynamically between non-SRQ and SRQ mode.

Reported-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>