USB: gadget: f_fs: even zero-length packets require a buffer

Some UDC drivers fails to queue a request if req->buf == NULL even for
ZLP requests. This patch adds a poisoned pointer instead of NULL to
make the code compliant with the gadget specification and catches
possible bug in the UDC driver if it tries to dereference buffer pointer
on ZLP request.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>