r6040: restore MIER register correctly when IRQ line is shared

When the r6040 device IRQ line is shared we will enter the driver
interrupt service routine, mask off the device interrupt enable
register (MIER) and return with IRQ_NONE, we would then leave the
device with interrupts disabled, this patch fixes that issue.

Reported-by: Steve Holland <sdh4@iastate.edu>
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>