light: tsl2591: simplify tsl2591_persist functions via lookup table

light: tsl2591: simplify tsl2591_persist functions via lookup table

Replace switch statements with an indexed lookup table for
persist cycle conversions.

Both functions contain redundant switch statements. This
reduces code duplication and makes future updates to
TSL2591_PRST_ALS_INT_CYCLE_* definitions easier to maintain
by keeping the mapping in a single place.

Signed-off-by: Matheus Silveira <matheus.feitosa@usp.br>
Co-developed-by: Lucas Rabaquim <lucas.rabaquim@usp.br>
Signed-off-by: Lucas Rabaquim <lucas.rabaquim@usp.br>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>