RDMA/efa: Properly document the interrupt mask register

The fact that the LSB in the register is the enable bit should not be an
implicit assumption between the driver and the device, properly document
that in the register definition.

Link: https://lore.kernel.org/r/20200225114010.21790-3-galpress@amazon.com


Reviewed-by: Firas JahJah <firasj@amazon.com>
Reviewed-by: Yossi Leybovich <sleybo@amazon.com>
Signed-off-by: Gal Pressman <galpress@amazon.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>