drivers: pwm: clarify behavior
drivers: pwm: clarify behavior

As written the specification when both period and pulse are zero is
inconsistent: allowed behavior would be to drive the pin at constant
active or constant inactive, depending on which condition was checked
first.

Clarify that driving constant active level requres a non-zero pulse
equal to period.

Signed-off-by: Peter Bigot <a4112a160b558d5427af084318038e19a95979ee@nordicsemi.no>

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