CLKDEV: Fix clkdev return value for NULL clk case

clkdev may incorrectly cause a clkdev entry with a NULL clk to return
-ENOENT.  This is not the intention of this code; -ENOENT should only
be returned if the clock entry can not be found in the table.  Fix
this.

Reported-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>