ena: Make first_interrupt a uint8_t

We do not have atomic(9) routines for bools, and it is not guaranteed
that sizeof(bool) is 1.

This fixes the KASAN and KMSAN kernel builds, which fail because the
compiler refuses to silently cast a _Bool * to a uint8_t * when calling
the atomic(9) sanitizer interceptors.

Reviewed by: Dawid Górecki <dgr@semihalf.com>
MFC after: 2 weeks
Fixes: 0ac122c388d9 ("ena: Use atomic_load/store functions for first_interrupt variable")
Differential Revision: https://reviews.freebsd.org/D35683