HID: mcp2221: fix 'cast to restricted __le16' sparse warnings

Use (__force __le16) cast for adc_values le16_to_cpu conversion to correct following sparse
warnings:

drivers/hid/hid-mcp2221.c:950:32: sparse: sparse: cast to restricted __le16

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 960f9df7 ("HID: mcp2221: add ADC/DAC support via iio subsystem")
Signed-off-by: Matt Ranostay <matt.ranostay@konsulko.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>