memstick: change to new flag variable

The EXTRA_CFLAGS assignment in memstick/Makefile was not accomplishing
anything because this flag only has effect on sources at the same level
as the makefile (i.e., per directory). Since both core/ and host/ rely
on MEMSTICK_DEBUG, the subdir-ccflags-y variant seems to be the
appropriate choice.

Signed-off-by: matt mooney <mfm@muteddisk.com>
Acked-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>