amd-xgbe: IRQ names require allocated memory

When requesting an irq, the name passed in must be (part of) allocated
memory. The irq name was a local variable and resulted in random
characters when listing /proc/interrupts. Add a character field to the
xgbe_channel structure to hold the irq name and use that.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>