The .regfield member of struct sti_pwm_compat_data only holds a pointer
to the global array sti_pwm_regfields. Replace the few usages by
directly using this array and drop the member, saving a bit of memory
and a few pointer dereferences.

Link: https://lore.kernel.org/r/7ddb76ef49fd84a07713b46c65374cb51f3b4ac0.1710068192.git.u.kleine-koenig@pengutronix.deSigned-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>