[ARM] pxa: cpufreq-pxa2xx: fix DRI recomputation routine

This patch:
1) Simpifies the DRI recomputation routine by pulling out the common code
2) Fixes a bug in PXA27x DRI recomputation caused by incorrect parenthesis

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>