sfxge(4): make RxQ type data an union

The type is an internal interface. Single integer is insufficient
to carry RxQ type-specific information in the case of equal stride
super-buffer Rx mode (packet buffers per bucket, maximum DMA length,
packet stride, head of line block timeout).

Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18234