ARM: OMAP: hwmod: fix an incorrect clk type cast with _get_clkdm

If the main clock for a hwmod is of basic clock type, it is illegal to type
cast this to clk_hw_omap and will result in bogus data. Fixed by checking
the clock flags before attempting the type cast.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Tested-by: Nishanth Menon <nm@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>