usart_windows: Use csp_windows_thread_create()
usart_windows: Use csp_windows_thread_create()

usart_windows.c is Windows dependent driver.  We can safely call
windows specific function, csp_windows_thread_create().

Signed-off-by: Yasushi SHOJI <afa9ca3be149447780c2f7b524d61ed3d3db63f9@spacecubics.com>

libcsp/libcsp,libcsp/libcsp