fix: kw41z: Use correct mapping for dBm

The radio API expects the setting of the TX-power in dBm. The actual
TX-power is however set by a 6bit register, which mapping between
register value and power in dBm is a lookup-table in the datasheet.

This mapping for the kw41z was off, which not only lead to incorrect
output power, but also to a maximal output power of only -1.9 dBm
instead of the possible +3.5 dBm.

Signed-off-by: Tobias Aschenbrenner <tobias.aschenbrenner@blik.io>
Signed-off-by: Franco Saworski <f.saworski@posteo.de>