USB: bcma: separate code initializing USB 2.0 core

This splits one big probing function into two smaller ones. The main one
is now responsible for the generic stuff: allocating memory & enabling
power using GPIO. The new one contains code that is specific to the USB
2.0 bcma core.
This will allow adding support for the USB 3.0 bcma core (handling XHCI)
in the future.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>