OMAP3 clock: Check return values for clk_get()

This patch checks if clk_get() returned success for
the clocks used in function omap2_clk_arch_init().

This version incorporates review comments from
Kevin Hilman and Paul Walmsley.

Signed-off-by: Sanjeev Premi <premi@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>