iio: adc/imx25-gcq: Move incorrect do_div

The newly added driver uses do_div() to device a 32-bit number, which now
provokes a warning:

drivers/iio/adc/fsl-imx25-gcq.c: In function 'mx25_gcq_setup_cfgs':
include/asm-generic/div64.h:207:28: warning: comparison of distinct pointer types lacks a cast
  (void)(((typeof((n)) *)0) == ((uint64_t *)0)); \

This replaces the do_div() call with a straight division operator.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 6df2e98c ("iio: adc: Add imx25-gcq ADC driver")
Signed-off-by: Lee Jones <lee.jones@linaro.org>