iio: adc: MCP3564: fix warn: unsigned '__x' is never less than zero.

The patch 33ec3e5f: "iio: adc: adding support for MCP3564 ADC"
leads to the following Smatch static checker warning:

   smatch warnings:
   drivers/iio/adc/mcp3564.c:1105 mcp3564_fill_scale_tbls() warn: unsigned '__x' is never less than zero.

vim +/__x +1105 drivers/iio/adc/mcp3564.c

   1094
   1095  static void mcp3564_fill_scale_tbls(struct mcp3564_state *adc)
   1096  {
   .....
   1103          for (i = 0; i < MCP3564_MAX_PGA; i++) {
   1104                  ref = adc->vref_mv;
 > 1105                  tmp1 = shift_right((u64)ref * NANO, pow);
   1106                  div_u64_rem(tmp1, NANO, &tmp0);
   1107
   .....
   1113  }

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202309280738.NWjVfVt4-lkp@intel.com/


Fixes: 33ec3e5f (iio: adc: adding support for MCP3564 ADC)
Signed-off-by: Marius Cristea <marius.cristea@microchip.com>
Link: https://lore.kernel.org/r/20231013132333.10582-1-marius.cristea@microchip.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>