[SCSI] Clean up scsi_add_lun a bit

This patch tidies up scsi_add_lun a bit.  I rewrote the kerneldoc to match
the actual parameters, moved the check for RBC and MMC REPORT_LUN devices
away from the switch(), changed the setup of sdev->type to account for
BLIST_ISROM, moved the check for BLIST_NO_ULD_ATTACH further down in
the function, removed a bogus comment and fixed some whitespace issues.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>