ena: fix building in-kernel driver

ena: fix building in-kernel driver

When building ENA as compiled into the kernel, the driver would fail to
build. Resolve the problem by introducing the following changes:
1. Add missing `ena_rss.c` entry in `sys/conf/files`.
2. Prevent SYSCTL_ADD_INT from throwing an assert due to an extra
CTLTYPE_INT flag.

Fixes: 986e7b92276 ("ena: Move RSS logic into its own source files")
Fixes: 6d1ef2abd33 ("ena: Implement full RSS reconfiguration")
Obtained from: Semihalf
Sponsored by: Amazon, Inc.
MFC after: 1 week