omap2 clock: use custom osc_ck enable/disable routines

Now that we're paying attention to the .enable/.disable fields in struct
clk, convert the osc_ck enable and disable code to use this standard
mechanism.  (osc_ck enable/disable does not technically enable or disable
the osc_ck; it actually enables/disables autoidling.)

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