power: reset: ltc2952: reduce dependency on global variables

Documentation/CodingStyle ch.4 mentions in a side node that global
variables should only be used if you really need them. Reduce the use of
the global instance of ltc2952_poweroff so we may eventually remove it
entirely.

While at it, rename ltc2952_poweroff_data to ltc2952_poweroff, just to
save that little bit of typing.

Signed-off-by: Frans Klaver <frans.klaver@xsens.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>