usb: fotg210: Compile into one module

It is since ages perfectly possible to compile both of these
modules into the same kernel, which makes no sense since it
is one piece of hardware.

Compile one module named "fotg210.ko" for both HCD and UDC
drivers by collecting the init calls into a fotg210-core.c
file and start to centralize things handling one and the same
piece of hardware.

Stub out the initcalls if one or the other part of the driver
was not selected.

Tested by compiling one or the other or both of the drivers
into the kernel and as modules.

Cc: Fabian Vogt <fabian@ritter-vogt.de>
Cc: Yuan-Hsin Chen <yhchen@faraday-tech.com>
Cc: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20221023144708.3596563-2-linus.walleij@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>