usb: gadget zero: avoid unnecessary reinit of data in f_sourcesink

In the IN case, since the USB request is allocated only when
the source/sink function is started and never freed, the USB ept
buffer needs to be initialized only at the beginning. This change
results into a more performant g_zero module, especially when
'pattern=1' is selected.

Signed-off-by: Armando Visconti <armando.visconti@st.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>