pwm: stm32: Fix error checking for a regmap_read() call

pwm: stm32: Fix error checking for a regmap_read() call

Without first assigning ret, it always evaluates to zero because
otherwise this code isn't reached. So assign the return code of
regmap_read() to ret to make the following error path do something.

This issue was spotted by Coverity.

Reported-by: Kees Bakker <kees@ijzerbout.nl>
Link: https://lore.kernel.org/linux-pwm/b0199625-9dbb-414b-8948-26ad86fd2740@ijzerbout.nl
Fixes: deaba9cff809 ("pwm: stm32: Implementation of the waveform callbacks")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/20241003114216.163715-2-u.kleine-koenig@baylibre.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>