usb: dwc2: gadget: Set the default EP max packet value as 8 bytes

Set the default EP max packet value as 8 bytes, because in the case
of low-speed, 'ep_mps' is not set. Thus, the default value of 'ep_mps'
should be considered for the case of low-speed.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>