[SCSI] arcmsr: fix write to device check

Use command->sc_data_direction instead of trying (incorrectly) to
figure it out from the command itself

[jejb: fix up compile failure]
Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: NickCheng <nick.cheng@areca.com.tw>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>