test: pwm: Remove range check when PWM signal has inverted polarity

There shall be no special case for inverted PWM pulse when checking
if the captured value is correct.

The output signal inversion is done in HW (at least for stm32h7xx)
SoC. The measured value of the pulse shall be the same - no matter
if we invert the signal or not (as measuring also takes into account
the inversion of the output/input signal).

Signed-off-by: Lukasz Majewski <lukma@denx.de>