sfxge(4): guard Rx scale code with corresponding option

Previously only some of the code was guarded by this which caused
a build error when EFSYS_OPT_RX_SCALE is 0 (e.g. in manftest).

Submitted by: Tom Millington <tmillington at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18280