clk: divider: return real rate instead of divider value

Commit bca9690b ("clk: divider: Make generic for usage elsewhere")
returned only the divider value for read-only dividers instead of the
actual rate.

Fixes: bca9690b ("clk: divider: Make generic for usage elsewhere")
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Tested-by: James Hogan <james.hogan@imgtec.com>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Michael Turquette <mturquette@linaro.org>