fbdev: ep93xx-fb: fix return value check in ep93xxfb_probe

function clk_prepare_enable may fail in ep93xxfb_probe, therefore,
add a return value check to clk_prepare_enable and handle the
error.

Signed-off-by: Yuanjun Gong <ruc_gongyuanjun@163.com>
Signed-off-by: Helge Deller <deller@gmx.de>