phy: renesas: phy-rcar-gen2: Fix the array off by one warning

Fix the below smatch warning by adding variable check rather than the
hardcoded value.
warn: array off by one? 'data->select_value[channel_num]'

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>