xillybus: Move out of staging

This driver has been functional and stable throughout the year it has spent
in the staging area. It has been patched for minor bugs, coding style issues
and improvements during this period.

This is the second submission of this move-out, after making several style
improvements, as suggested by Dan Carpenter.

Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>