dev is never assigned or used. Remove it.

Fixes: 7edf7369 ("pwm: Add driver for STM32 plaftorm")
Signed-off-by: NFabrice Gasnier <fabrice.gasnier@st.com>
Reviewed-by: NBenjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: NThierry Reding <thierry.reding@gmail.com>