[SCSI] hpsa: eliminate 8 external target limitation

Driver limits SAS external target IDs to range 1-8.
Need to increase limit and clean up overlapping concepts of targets and paths
in the code.

There are several defined constants that control this:
HPSA_MAX_TARGETS_PER_CTLR     16
MAX_MSA2XXX_ENCLOSURES        32
HPSA_MAX_PATHS                8

We can condense this to one constant:
MAX_EXT_TARGETS               32

SAS switches allow for 8 connections, and there is capacity for 4 switches per
enclosure in largest blade enclosure type.

Signed-off-by: Scott Teel <scott.teel@hp.com>
Signed-off-by: Stephen M. Cameron <scameron@beardog.cce.hp.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>