fsl: serdes: ensure accessing the initialized maps of serdes protocol

Up to now, the function is_serdes_configed() doesn't check if the map
of serdes protocol is initialized before accessing it. The function
is_serdes_configed() will get wrong result when it was called before
the serdes protocol maps initialized. As the first element of the map
isn't used for any device, so use it as the flag to indicate if the
map has been initialized.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>