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

csp_if_udp.c is already posix specific.  Replace the wrapper function
with native pthread_create.

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

libcsp/libcsp,libcsp/libcsp