[ath] initialise do_ldpc to 0.

I .. can't believe I missed this.

This showed up because the AP was TX'ing LDPC to an iwm(4) chipset,
which didn't advertise LDPC and doesn't /accept/ LDPC. Amusingly, all
the two other FreeBSD 11n parts I had tested with (AR9380, Intel 7260)
and I completely forgot to test on ye olde hardware.

That'll teach me.

Tested:

• AR9580 (AP) - Intel 7260 (STA), AR9380 (STA), Intel 6205 (STA)