r6040: save and restore MIER correctly in the interrupt routine

This patch saves the MIER register contents before treating
interrupts, then restores them correcty at the end of the
interrupt routine.

Signed-off-by: Joe Chou <Joe.Chou@rdc.com.tw>
Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>