pinctrl: stm32: remove useless check

There is no link between the number of elements of tab which contains all
pin desc (located in each pinctrl-stm32xxxx.c files) and the pin number
(defined in the tab).

Signed-off-by: Alexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>