clk: clk-wm831x: fix a logic error

Fix bug https://bugzilla.kernel.org/show_bug.cgi?id=188561

. Function
wm831x_clkout_is_prepared() returns "true" when it fails to read
CLOCK_CONTROL_1. "true" means the device is already prepared. So
return "true" on the read failure seems improper.

Signed-off-by: Pan Bian <bianpan2016@163.com>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Fixes: f05259a6 ("clk: wm831x: Add initial WM831x clock driver")
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>