[SCSI] scsi_transport_sas: make write attrs writeable

A couple write attributes in sas transport layer have a small
bug that prevents them from being written to.  Those
attributes are the link_reset and write_reset.  This is due
the store field being set to NULL.

Signed-off-by: Eric Moore <Eric.Moore@lsil.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>