OMAP clock: drop clk_get_usecount()

This function is race-prone and mistakenly conveys the impression to
drivers that it is part of the clock interface.  Get rid of it: core
code that absolutely needs this can just check clk->usecount.  Drivers
should not use it at all.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>