bxe(4): Use CTLFLAG_RDTUN flag definition

sysctl variables rx_budget and max_aggregation_size are read-only loader
tunable. Mark them with CTLFLAG_RD flag.

No functional change intended.

Reviewed by: hselasky, erj
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D39466