usb: gadget: f_fs: Use config_ep_by_speed()

In commit 2bfa0719 ("usb: gadget: function: f_fs: pass
companion descriptor along") there is a pointer arithmetic
bug where the comp_desc is obtained as follows:

 comp_desc = (struct usb_ss_ep_comp_descriptor *)(ds +
	       USB_DT_ENDPOINT_SIZE);

Since ds is a pointer to usb_endpoint_descriptor, adding
7 to it ends up going out of bounds (7 * sizeof(struct
usb_endpoint_descriptor), which is actually 7*9 bytes) past
the SS descriptor. As a result the maxburst value will be
read incorrectly, and the UDC driver will also get a garbage
comp_desc (assuming it uses it).

Since Felipe wrote, "Eventually, f_fs.c should be converted
to use config_ep_by_speed() like all other functions, though",
let's finally do it. This allows the other usb_ep fields to
be properly populated, such as maxpacket and mult. It also
eliminates the awkward speed-based descriptor lookup since
config_ep_by_speed() does that already using the ones found
in struct usb_function.

Fixes: 2bfa0719 ("usb: gadget: function: f_fs: pass companion
descriptor along")
Cc: stable@vger.kernel.org
Change-Id: I9121f4df898de1455f9be4333ca8d0e744f4eb9e
Signed-off-by: Jack Pham <jackp@codeaurora.org>