usb: gadget: f_fs: Fix enumeration in fullspeed mode

ffs_func_eps_enable always take the highspeed usb_endpoint_descriptor
if it is available. Choose the correct usb_endpoint_descriptor
depending on the gadget speed instead.

Change-Id: I5b8444cfdf1b6fe1eef25525fb7a0a0f617a353e
Signed-off-by: Benoit Goby <benoit@android.com>