[SCSI] make supported_mode default to initiator.

if shost->supported mode is zero (i.e. MODE_UNKNOWN) show it as
initiator (it's obviously an unconverted driver that won't do target).

Acked-by: FUJITA Tomonori <tomof@acm.org>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>