ath_hal_ar9300: implement the TX/RX chainmask override for AR9300 HAL

This commit implements the missing capability set handlers for
setting the transmit and receive chainmasks. I did this for the
AR5416 and later chips years ago, but not for these.

This is especially useful when you're only hooking up one or two
antennas on a 3 antenna device - or you just want to test it like
that.

It also requires updating the number of supported transmit/receive
streams, so also add them here.

Locally Tested:

• AR9300, STA mode, 1, 2 and 3 stream TX/RX configuration

Differential Revision: https://reviews.freebsd.org/D48240