Fix compilation warnings when building ENA driver with gcc compiler

[mk_semihalf.com]

Summary

The gcc compiler is more sensitive when variable is having an value
assigned, but it is not used anywhere further.

[rlibby]

sys/dev/ena/ena.c

2765–2766 ↗

(On Diff #34529)

Nothing wrong with this formulation, but an alternative would have been just to delete the ena_dev variable, since it is otherwise pointless. ena_handle_msix and ena_unmask_all_io_irqs appear to have gone with that approach.