pinctrl: st: don't specify default interrupt trigger

Thanks to 332e99d5 which now alerts of default
trigger usage when configuring interrupts.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Acked-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>