[ARM] omap: fix clockdomain enable/disable ordering

Based on a patch from Paul Walmsley <paul@pwsan.com>:

 omap2_clk_enable() should enable a clock's clockdomain before
 attempting to enable its parent clock's clockdomain.  Similarly, in
 the unlikely event that the parent clock enable fails, the clockdomain
 should be disabled.

 linux-omap source commit is 6d6e285e5a7912b1ea68fadac387304c914aaba8.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>