clk: divider: Fix clk_divider_round_rate() to use clk_readl()

clk-divider uses clk_readl()/clk_writel() everywhere, except in
clk_divider_round_rate(), where plain readl() is used. Change this to
clk_readl(), as it makes a difference on powerpc.

Fixes: e6d5e7d9 ("clk-divider: Fix READ_ONLY when divider > 1")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>