pwm: spear: Fix checking return value of clk_enable() and clk_prepare()

The logic to check return value of clk_enable() and clk_prepare() is reversed,
fix it.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Cc: stable@vger.kernel.org
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>