usb: misc: usb3503: Factor out I2C probe

In preparation for supporting operation without an I2C control interface
factor out the I2C-specific parts of the probe routine from those that
don't do any register I/O.

Signed-off-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>