turn proper error if clk_get fails

Return PTR_ERR(sfb->bus_clk) instead of 0 if clk_get fails.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>
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