clk: pxa: fix pxa2xx_determine_rate return

The new pxa2xx_determine_rate() function seems lacking in a few
regards:

- For an exact match or no match at all, the rate is uninitialized
  as reported by gcc -Wmaybe-unintialized:
   drivers/clk/pxa/clk-pxa.c: In function 'pxa2xx_determine_rate':
   drivers/clk/pxa/clk-pxa.c:243:5: error: 'rate' may be used uninitialized in this function

- If we get a non-exact match, the req->rate output is never set
  to the actual rate but remains at the requested rate.

- We should not attempt to print a rate if none could be found

This rewrites the logic accordingly.

Fixes: 9fe69429 ("clk: pxa: transfer CPU clock setting from pxa2xx-cpufreq")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>