[SCSI] megaraid: Add support for change_queue_depth

Adds support for change_queue_depth so that device
queue depth can be changed at runtime through sysfs.

Signed-off-by:  <brking@charter.net>
Acked-by: Seokmann Ju <seokmann.ju@lsil.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>