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