drivers: usb_dc_stm32: Fix OUT transfer issue

The driver cannot handle OUT transactions for an endpoint with an
MPS smaller than 64 bytes. To solve the issue, we will not use one
fixed value, EP_MPS, but instead use the actual MPS of an endpoint,
ep_state->ep_mps.

Signed-off-by: Marc Desvaux <marc.desvaux-ext@st.com>
Signed-off-by: Loic Poulain <loic.poulain@linaro.org>