lib: Inadvertent assignment in ? operator

lib: Inadvertent assignment in ? operator

In the case where we are using select as
the operator *and* we call
funcname_thread_add_read_write *and* the
fd is already set, we would overwrite
the read/write direction to always be READ.

Clearly this was a bad idea.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>