Move I2C code into its own source file. Makes the mode-setting
code a little less convoluted.

v3:
	* fix SPDX tag to say 'SPDX-License-Identifier'
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20211206091125.29501-4-tzimmermann@suse.de