net/ena/base: use 48-bit memory addresses

net/ena/base: use 48-bit memory addresses

ENA device is using 48-bit memory for IO. Because of that, the upper
limit had to be updated.

From the driver perspective, it's just a cosmetic change to make
definition of the structure 'ena_common_mem_addr' more descriptive and
the address value was verified anyway for the valid range in the
function 'ena_com_mem_addr_set()'.

Signed-off-by: Michal Krawczyk <mk@semihalf.com>
Reviewed-by: Igor Chauskin <igorch@amazon.com>
Reviewed-by: Guy Tzalik <gtzalik@amazon.com>