power: reset: replace curly brackets in Makefile

Normal parentheses should be used when referring to config variables
in Makefile. Replace the accidentally introduced curly brackets by
regular parentheses.

Fixes: a7f79f99541ef ("power: reset: add driver for LinkStation power off")
Acked-by: Daniel González Cabanelas <dgcbueu@gmail.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>