[MIPS] Malta: check the PCI clock frequency in a separate function

This patch adds a separate short and sweet function to check the
PCI clock frequency. This is to improve readability of the Malta
setup code.

Along the way, a couple of coding style violations are fixed.

No functional changes introduced.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>