pwm: sti: Improve error reporting using dev_err_probe()

pwm: sti: Improve error reporting using dev_err_probe()

This has the advantage of handling EPROBE_DEFER correctly and being more
compact.

This change also introduces an error message for a few error paths that
lacked an error indicator before. Also sti_pwm_probe_dt() is renamed to
sti_pwm_probe_regmap() to better fit what it actually does.

Link: https://lore.kernel.org/r/8e540733ab882f2b8873712faf85c4f0cb48133a.1710068192.git.u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>