leds: pca9532: correct shift computation in pca9532_getled

Each led setting occupies two bits in a corresponding led register.
Accessing these bits requires shifting and masking, which was
implemented incorrectly in pca9532_getled. Two new helper macros
concentrate the computation of those masks in one place.

Signed-off-by: Markus Moll <mmoll@de.pepperl-fuchs.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>