clk: renesas: rcar-gen3: Remove superfluous masking in cpg_z_clk_set_rate()

Due to the clamping of mult, "(32 - mult) << __ffs(zclk->mask)" can
never exceed the mask.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Tested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/20210326120100.1577596-4-geert+renesas@glider.be