platform/mulle: Clean up power-control.

Tidied up the header macros to make it less likely to have typos when
modifying for custom hardware.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
platform/mulle: Add sleep inhibitor to radio driver.

Prevents LLS mode when radio is turned on in order to avoid a scenario
where the MCU is not woken by the radio when a packet has arrived.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>