FRV: Enable the MB86943 PCI arbiter correctly

Enable the MB93090 motherboard's MB86943 PCI arbiter correctly by assigning to
the register rather than comparing against it.  This is required to support
bus mastering.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>