cpu/nrf52: Fix PWM generation if <4 channels are used
cpu/nrf52: Fix PWM generation if <4 channels are used

The PWM decodeer, in LOAD=Single mode, loads data in groups of 4
half-words. Previously, if any channel was not connected to a pin (a
connection that's immaterial to the loader), a smaller number would be
given resulting in the decoder not loading anything at all.

See-Also: https://infocenter.nordicsemi.com/pdf/nRF52840_PS_v1.1.pdf#page=256

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