[MIPS] Malta: Fix reading the PCI clock frequency on big-endian

The JMPRS register on Malta boards keeps a 32-bit CPU-endian
value. The readw() function assumes that the value it reads is a
little-endian 16-bit number. Therefore, using readw() to obtain
the value of the JMPRS register is a mistake. This error leads
to incorrect reading of the PCI clock frequency on big-endian
during board start-up.

Change readw() to __raw_readl().

This was tested by injecting a call to printk() and verifying
that the value of the jmpr variable was consistent with current
setting of the JP4 "PCI CLK" jumper.

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