amd-xgbe: Unify coherency checking logic with device_dma_is_coherent()

Currently, amd-xgbe driver has separate logic to determine device
coherency for DT vs. ACPI. This patch simplifies the code with
a call to device_dma_is_coherent().

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>