drivers: usb: core: hcd.c: converted busmap from struct to bitmap

The DECLARE_BITMAP macro should be used for declaring this bitmap.
This commit converts the busmap from a struct to a simple (static)
bitmap, using the DECLARE_BITMAP macro from linux/types.h.

Please review, as I'm new to kernel development, I don't know if this
has any hidden side effects!

Suggested by joe@perches.com

Signed-off-by: Matthias Beyer <mail@beyermatthias.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>