ARM: davinci: clock: Correct return values for API functions

Fix the values returned by the publicly used functions.
These function should return 0 when they are called with clk == NULL in
similar manner as the clock API does to avoid different behavior in drivers
used by daVinci and other architectures.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>