iio: adc: mcp320x: Tidy up endian types in type cast.

Fixes the sparse warning:
drivers/iio/adc/mcp320x.c:167:41: warning: incorrect type in argument 1 (different base types)
drivers/iio/adc/mcp320x.c:167:41:    expected restricted __be32 const [usertype] *p
drivers/iio/adc/mcp320x.c:167:41:    got unsigned int [usertype] *<noident>

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Lukas Wunner <lukas@wunner.de>