tests: drivers: pwm: Revert PWM capture change for pulse check

Reverts a change regarding how the test checks the pulse output
from a capture event. A pulse capture will produce a value that has
the same polarity (and range) as programmed by pwm_capture().
Checking 'pulse_capture' against 'period - pulse' means expecting
the driver to return a capture value that is not inverted as well,
which would cause test cases to fail.

Signed-off-by: Raffael Rostagno <raffael.rostagno@espressif.com>