iio:adc:qcom-spmi-vadc : fix undefined __divdi3

A simple do_div call works here as all the signed 64 bit is
actually small and unsigned at this point, and the numerator is
u32.

Introduce a temporary u64 variable to avoid type comparison warnings
on some architectures.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Rama Krishna Phani A <rphani@codeaurora.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>