usb: misc: usb3503: Factor out I2C probe

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>
[mszyprow: mainline commit 2487e3ee33dd6c4fa3dabbe11bc988883be81f1e]
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Change-Id: I2487e3ee33dd6c4fa3dabbe11bc988883be81f1e