usb-uas: set max_lun and max_channel

256 luns is what the sam-4 address method 0 can handle and what
the qemu uas emulation supports.  So pick that for now.

[ v2: unlike the other two max_* fields max_channel isn't max+1 ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>