fbdev: mmp: fix value check in mmphw_probe()

in mmphw_probe(), check the return value of clk_prepare_enable()
and return the error code if clk_prepare_enable() returns an
unexpected value.

Fixes: d63028c3

 ("video: mmp display controller support")
Signed-off-by: Yuanjun Gong <ruc_gongyuanjun@163.com>
Signed-off-by: Helge Deller <deller@gmx.de>