sc_fixed_ridx is initialised to 0 but tested as it it were initialised to -1.  The result of this is tx frames were always sent out at fixed rate 0 instead of ni_txrate.
sc_fixed_ridx is initialised to 0 but tested as it it were initialised
to -1.  The result of this is tx frames were always sent out at fixed
rate 0 instead of ni_txrate.

Match the iwn behaviour and test ic_fixed_rate for -1 instead.
Problem spotted by kettenis@ in an earlier diff.

ok kettenis@ stsp@

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars