net: renesas: rswitch: Move linkfix variables to rswitch_gwca

To improve readability, move linkfix related variables to
struct rswitch_gwca. Also, rename function names "desc" with "linkfix".

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>