Input: move fixp-arith.h to drivers/input

Move fixp-arith.h from drivers/usb/input to drivers/input, as the part of
force feedback support that requires trigonometric functions is being moved
there.

Signed-off-by: Anssi Hannula <anssi.hannula@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>