pwm: Check for CONFIG_PWM using IS_REACHABLE() in main header

Preparing CONFIG_PWM becoming tristate the right magic to check for the
availability of the pwm functions is using IS_REACHABLE() and not
IS_ENABLED(). The latter gives the wrong result for built-in code with
CONFIG_PWM=m.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/20250217102504.687916-2-u.kleine-koenig@baylibre.com


Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>