mpc512x/clock: fix clk_get logic

[Mark Brown]

On Fri, Oct 30, 2009 at 10:17:14AM +0100, Wolfram Sang wrote:
> The matching logic returns a clock even if only the dev-part matches. This is
> wrong as devices may utilize more than one clock, so the wrong clock may be
> returned due to dev being not unique (noticed while working on the CAN driver).
> The proposed new method will:

> - require the id field (as _this_ is the unique identifier)

NULL id fields are supposed to be supported in the cannonical clkdev
API, unfortunately.