thermal: stm32: fix spelling mistake "preprare" -> "prepare"

There is a spelling mistake in a dev_err error message. Fix it.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200130100537.18069-1-colin.king@canonical.com