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: NScott Teel <scott.teel@hp.com>
Signed-off-by: NStephen M. Cameron <scameron@beardog.cce.hp.com>
Signed-off-by: NJames Bottomley <JBottomley@Parallels.com>