net: ena: use SHUTDOWN as reset reason when closing interface

The 'ENA_REGS_RESET_SHUTDOWN' enum indicates a normal driver
shutdown / removal procedure.

Also, a comment is added to one of the reset reason assignments for
code clarity.

Signed-off-by: Shay Agroskin <shayagr@amazon.com>
Signed-off-by: Arthur Kiyanovski <akiyano@amazon.com>
Signed-off-by: Sameeh Jubran <sameehj@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>