mfd: sun4i-gpadc: Fix 'cast from pointer to integer of different size' warning

When building for X86 using COMPILE_TEST we get this warning:

../drivers/mfd/sun4i-gpadc.c: In function ‘sun4i_gpadc_probe’:
../drivers/mfd/sun4i-gpadc.c:110:10: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]

Since an integer and a pointer are difference sizes on 64bit architectures.

Convert to case to a long instead.

Signed-off-by: Lee Jones <lee.jones@linaro.org>