gre: simplify RSS ifdefs

Use the early break to avoid else definitions. When RSS gains a
runtime option previous constructs would duplicate and convolute
the existing code.

While here init flowid and skip magic numbers and late default
assignment.

Reviewed by: melifaro, kbowling
Obtained from: OPNsense
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31584

(cherry picked from commit bb250fae9e9e278b681cf3a71ced718700ecf74c)