ARM: OMAP2+: Fix error handling for omap2_clk_enable_init_clocks

We need to check if we got the clock before trying to do anything
with it. Otherwise we will get something like this:

Unable to handle kernel paging request at virtual address fffffffe
...
[<c04bef78>] (clk_prepare) from [<c00338a4>] (omap2_clk_enable_init_clocks+0x50/0x8)
[<c00338a4>] (omap2_clk_enable_init_clocks) from [<c0876838>] (dm816x_dt_clk_init+0)
...

Let's add check for the clock and WARN if the init clock was not
found.

Cc: Brian Hutchinson <b.hutchman@gmail.com>
Cc: Paul Walmsley <paul@pwsan.com>
Cc: Tero Kristo <t-kristo@ti.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>