pinctrl: zte: fix dereference of 'data' in zx_set_mux()

It fixes the following Smatch complaint:

drivers/pinctrl/zte/pinctrl-zx.c:76 zx_set_mux()
warn: variable dereferenced before check 'data' (see line 67)

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: cbff0c4d ("pinctrl: add ZTE ZX pinctrl driver support")
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>