Remove unused argument from static function in ena.c

The function ena_enable_msix_and_set_admin_interrupts takes two
arguments while the second is not used and so can be spared. This is a
static function, only ena.c is affected.

Submitted by: Maciej Bielski <mba@semihalf.com>
Obtained from: Semihalf
Sponsored by: Amazon, Inc.