fix(drivers/usb): remove deadcode when USBD_EP_NB = 1

CID 373791:  Control flow issues  (DEADCODE)
CID 373789:  Control flow issues  (DEADCODE)

Since USBD_EP_NB = 1 for DFU stack on STMP32MP15 platform (only EP0 is
required for DFU support) the value of num can't be different of 0
and the code can't be reached in usb_core_receive / usb_core_transmit.

Add a simple sub-function with this part of code.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Change-Id: I07a56909bb1e6de19ce52da7945b6d2916be8538
1 file changed