power: supply: bq27xxx: move overtemp tests to a switch statement.

This is done for readability as the upcoming commits will add a lot of
cases.

tested: no

Signed-off-by: Chris Lapa <chris@lapa.com.au>
Acked-by: Pali Rohár <pali.rohar@gmail.com>
Reviewed-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>