rum(4): add error handling for rum_enable_tsf_sync() and rum_prepare_beacon()

Tested:

• rum0: MAC/BBP RT2573 (rev 0x2573a), RF RT2528, STA mode Note: haven't tested AP mode yet; will do once the rest of the AP mode / power save commits are in.

Submitted by: <s3erios@gmail.com>
Differential Revision: https://reviews.freebsd.org/D3624