[SCSI] hpsa: factor out tail calls to next_command() in process_(non)indexed_cmd()

This is in order to smooth the way for upcoming changes to allow use of
multiple reply queues for command completions.

Signed-off-by: Stephen M. Cameron <scameron@beardog.cce.hp.com>
Signed-off-by: Matt Gates <matthew.gates@hp.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>