rt2800: fix TX_PIN_CFG setting for non MT7620 chips

Since commit 41977e86 ("rt2x00: add support for MT7620") we do not
initialize TX_PIN_CFG setting. This cause breakage at least on some
RT3573 devices. To fix the problem patch restores previous behaviour
for non MT7620 chips.

Fixes: 41977e86 ("rt2x00: add support for MT7620")
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1480829


Reported-and-tested-by: Jussi Eloranta <jussi.eloranta@csun.edu>
Cc: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>