[MIPS] Alchemy: fix off by two error in __fixup_bigphys_addr()

the PCI specific code in this function doesn't check for the address range
being under the upper bound of the PCI memory window correctly -- fix this,
somewhat beautifying the code around the check, while at it...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>