rum(4): reduce code duplication.

Tested:

rum0: <Belkin Belkin 54g USB Network Adapter, class 0/0, rev 2.00/0.01, addr 21> on usbus0
rum0: MAC/BBP RT2573 (rev 0x2573a), RF RT2528

Submitted by: <s3erios@gmail.com>
Differential Revision: https://reviews.freebsd.org/D3606