ARM: OMAP2+: PM debug: fix the use of debugfs_create_* API

Check the return code pointer value from debugfs_create_dir for error
or NULL.
Also added an additional check to prevent the creation of a 'suspend'
entry at the debugfs root in case a power domain directory cannot be
created.

Signed-off-by: Jean Pihet <j-pihet@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>