drivers: led_strip: tlc5971: fix brightness control bit shifting

The lower bits of the brightness control registers were erroneously
shifted even though FIELD_PREP already does the shifting. In practice,
the effect is negliglible but of course should be fixed.

Also add missing parenthesis around shifting operations to satisy static
analysis.

Signed-off-by: Mikkel Jakobsen <mikkel.aunsbjerg@escolifesciences.com>