sbp-target: use simple assignment in tgt_agent_rw_agent_state()

There is no need to memcpy() a 32-bit integer. The data pointer is
guaranteed to be quadlet aligned by the FireWire stack so we can replace
the memcpy() with an assignment.

Thanks to Stefan Richter.

Signed-off-by: Chris Boot <bootc@bootc.net>
Cc: Stefan Richter <stefanr@s5r6.in-berlin.de>
Cc: Andy Grover <agrover@redhat.com>
Cc: Clemens Ladisch <clemens@ladisch.de>
Cc: Nicholas A. Bellinger <nab@linux-iscsi.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>