pinctrl: st: Fix Sparse error

This change fixes below sparse error,
drivers/pinctrl/pinctrl-st.c:1515:31: error: incompatible types for operation (>)
drivers/pinctrl/pinctrl-st.c:1515:31:    left side has type void [noderef] <asn:2>*irqmux_base
drivers/pinctrl/pinctrl-st.c:1515:31:    right side has type int

Cc: Maxime Coquelin <maxime.coquelin@st.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
CC: Linus Walleij <linus.walleij@linaro.org>
Cc: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Pramod Gurav <pramod.gurav@smartplayin.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>