staging: greybus: pwm: Make use of devm_pwmchip_alloc() function

This prepares the greybus pwm driver to further changes of the pwm core
outlined in the commit introducing pwmchip_alloc(). There is no intended
semantical change and the driver should behave as before.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/3206ab7f49c2c1704ea69446f3b7a7d1e71200fa.1707900770.git.u.kleine-koenig@pengutronix.de


Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>