[ARM] omap: fix usecount decrement bug

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

 If _omap2_clk_enable() fails, the clock's usecount must be decremented
 by one no matter whether the clock has a parent or not.

but reorganised a bit.

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