clk: rockchip: Remove an unused field in struct rockchip_mmc_clock

In "struct rockchip_mmc_clock", the 'id' field is unused.
Remove it.

Found with cppcheck, unusedStructMember.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/410bc0f86c7b9f1c80f8a4e9a2a028a9a6ee1ec0.1713970085.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Heiko Stuebner <heiko@sntech.de>