pwm: stmpe: Silence duplicate error messages

stmpe_reg_read() and stmpe_reg_write() already emit error messages when
they fail. So the extra error messages in the pwm driver are only little
useful. They are useful in some situation, as they give a bit of context
to the failing register write. So don't remove them but degrade them to
dev_dbg().

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