net: ena: unmask MSI-X only after device initialization is completed

Under certain conditions MSI-X interrupt might arrive right after it
was unmasked in ena_up(). There is a chance it would be processed by
the driver before device ENA_FLAG_DEV_UP flag is set. In such a case
the interrupt is ignored.
ENA device operates in auto-masked mode, therefore ignoring
interrupt leaves it masked for good.
Moving unmask of interrupt to be the last step in ena_up().

Signed-off-by: Netanel Belgazal <netanel@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>