IB/srpt: Do not accept invalid initiator port names

Make srpt_parse_i_port_id() return a negative value if hex2bin()
fails.

Fixes: commit a42d985b ("ib_srpt: Initial SRP Target merge for v3.3-rc1")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>