clk: samsung: Initialize clock table with error pointers

Before this patch, the driver was simply zeroing the clock table, which
is incorrect, because invalid clock numbers returned NULL instead of
error pointers. This patch fixes this by changing the driver to
initialize the array with PTR_ERR(-ENOENT).

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>