drivers: udc: udc_skeleton: fix call to K_THREAD_STACK_DEFINE

The udc_skeleton driver incorrectly calls `K_THREAD_STACK_DEFINE` with
`CONFIG_UDC_SKELETON` instead of `CONFIG_UDC_SKELETON_STACK_SIZE`. This
causes stack overflows when developing a driver based on this code.

Fix the call to `K_THREAD_STACK_DEFINE`.

Signed-off-by: Miguel Gazquez <miguel.gazquez@bootlin.com>