[SCSI] hpsa: improve naming on external target device functions

Reduce confusion and inaccuracy caused by dated naming of vars and functions
referring to external target devices.

CURRENT NAMING:                PROPOSED NAMING:

"MSA2xxx devices"              "external target devices"
msa2xxx_model                  ext_target_model
is_msa2xxx                     is_ext_target
add_msa2xxx_enclosure          add_ext_target_dev
nmsa2xxx_enclosures            n_ext_target_devs

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>