drivers: pwm: pwm_mcux: fix for coverity 321142

Fix `PWM_SetupPwm` function being called with `numOfChnls`
argument equal to 2, when in fact only one channel is being set up.

Also add 'U' suffix to `pwmFreq_Hz` unsigned integer argument in
the function call.

Signed-off-by: Gustavo Silva <gustavograzs@gmail.com>