We cannot select INTEL_IOATDMA in Kconfig as soon as MYRI10GE or IXGBE
is enabled since the former is not available on all architectures.

Just use a Kconfig bool {IXGBE,MYRI10GE}_DCA set to =y when DCA
support can actually be built.

[myri10ge portion written and signed-off-by] Brice Goglin <brice@myri.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>