HID: mcp2221: fix usage of tmp variable in mcp2221_raw_event()

In mcp2221_raw_event(), 'tmp' is used only conditionally. Move
the declaration into the conditional block in order to prevent
unused variable warning.

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