clk: versatile: mask VCO bits before writing

The Versatile syscon ICST driver OR:s the bits into place but
forgets to mask the previous value, making the code only work
if the register is zero or giving haphazard results. Mask the
19 bits used by the Versatile syscon interface register.

Regression caused and now fixed by yours truly.

Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: linux-clk@vger.kernel.org
Fixes: 179c8fb3 ("clk: versatile-icst: convert to use regmap")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>