USB: serial: refactor and clean up endpoint handling

Refactor and clean up endpoint handling.

This specifically moves the endpoint-descriptor arrays of the stack.

Note that an err_free_epds label is not yet added to avoid a compilation
warning when neither CONFIG_USB_SERIAL_PL2303 or
CONFIG_USB_SERIAL_GENERIC is selected.
Signed-off-by: Johan Hovold <johan@kernel.org>