atl1: add value to check ability of reenabling IRQs

Unfortunately it is not clear from code is usage of
IMR register possible or not. So, to prevent possible
side-effects of reading this register i prefer store
interrupts enable flag separately.

Signed-off-by: Tony Zelenoff <antonz@parallels.com>
Signed-off-by: David S. Miller <davem@davemloft.net>