rum(4): some non-functional changes / cleanup

• Remove unused sc_txtap_len/sc_rxtap_len fields.
• Remove unused ackrate variable.
• Remove unneded warning in rum_update_mcast().
• Use nitems().
• Replace some hardcoded values for RT2573_MAC_CSR1 register.
• Remove second argument for RUM_LOCK_ASSERT() - it is always the same.

Submitted by: <s3erios@gmail.com>
Differential Revision: https://reviews.freebsd.org/D3605