usart_linux: Use csp_posix_thread_create()
usart_linux: Use csp_posix_thread_create()

usart_linux.c is Linux dependent driver.  We can safely call Linux
dependent function, csp_posix_thread_create().

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

libcsp/libcsp,libcsp/libcsp