[SCSI] scsi_debug: create new scsi_debug devices at a single place

Two functions, sdebug_add_adapter and devInfoReg, creates new
scsi_debug devices. To simplify the code, this patch adds a new helper
function to create new scsi_debug devices (sdebug_device_create) and
converts both functions to use it.

I plan to add more to scsi_debug devices (e.g. using a thread for a
scsi_debug device for scalability testings). This patch enable me to
add such to just the new helper function instead of touching two
functions, sdebug_add_adapter and devInfoReg.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Douglas Gilbert <dougg@torque.net>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>