usb: gadget: f_fs: do not set cancel function on synchronous {read,write}

do not try to set cancel function in synchronous operations in
ffs_epfile_{read,write}_iter.

Cc: <stable@vger.kernel.org> # v4.0+
Acked-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>