csp_thread: posix: Replace thread_create in usart_linux.c
csp_thread: posix: Replace thread_create in usart_linux.c

usart_linux.c is already Linux specific usart driver.  Replace the
wrapper function with native pthread_create.

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

libcsp/libcsp,libcsp/libcsp