staging: pi433: fixed signedness of 4th argument to kfifo_from_user

sparse warning:
incorrect type in initializer (different signedness)
expected unsigned int *__copied, got int *<noident>

Signed-off-by: Stefano Manni <stefano.manni@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>