[ARM] 3965/1: ixp2000: fix handling of pci master aborts

The master abort check in ixp2000_pci_read_config() recently started
failing due to the compiler optimising out the read access following
the clearing of pci_master_aborts.  Mark pci_master_aborts volatile to
force the compiler to reload it on every use.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>