USB: Increase usbfs transfer limit

Promote a variable keeping track of USB transfer memory usage to a
wider data type and allow for higher bandwidth transfers from a large
number of USB devices connected to a single host.

Signed-off-by: Mateusz Berezecki <mateuszb@fastmail.fm>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>