pwm: Mark free pwm IDs as used in alloc_pwms()

alloc_pwms() only identified a free range of IDs and this range was marked
as used only later by pwmchip_add(). Instead let alloc_pwms() already do
the marking (which makes the function actually allocating the range and so
justifies the function name). This way access to the allocated_pwms
bitfield is limited to two functions only.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20221117211143.3817381-4-u.kleine-koenig@pengutronix.de


Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>