pch_phub: fix sparse warning

sparse warns about using 0 as NULL pointer,

drivers/misc/pch_phub.c:702:44: warning: Using plain integer as NULL pointer

Signed-off-by: Devendra Naga <develkernel412222@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>