pwm: spear: Simplify using devm functions

With devm_clk_get_prepared() the call to clk_unprepare() can be dropped
from the error path and the remove callback. With devm_pwmchip_add()
pwmchip_remove() can be dropped. Then the remove callback is empty and
can go away, too. With spear_pwm_remove() the last user of
platform_get_drvdata() is gone and so platform_set_drvdata() can be
dropped, too.

Also use dev_err_probe() for simplified (and improved) error reporting.

Link: https://lore.kernel.org/r/20230929161918.2410424-7-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>