ARM: omap2+: hwmod: fix potential NULL pointer access

omap_hwmod_get_pwrdm() may access a NULL clk_hw pointer in some failure
cases. Add a check for the case and bail out gracely if this happens.

Reported-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>
Cc: stable@vger.kernel.org # v5.10+
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>