IXP4xx: Fix compilation failure with CONFIG_IXP4XX_INDIRECT_PCI.

Instead of including the heavy linux/mm.h for VMALLOC_START, test the addresses
against PCI MIN and MAX addresses. Indirect PCI uses 1:1 mapping for MMIO space
making this change possible.

Signed-off-by: Krzysztof Hałasa <khc@pm.waw.pl>