[SCSI] srp_transport: convert to use supported_mode attribute

srp transport works for target drivers without supported_mode
attribute but it would be better to use it explicitly.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>