rt2800: enable TX_PIN_CFG_RFRX_EN only for MT7620

The TX_PIN_CFG_RFRX_EN bit was not set on other devices than MT7620,
restore old behavaviour since setting this bit maight not be
correct for older devices.

Fixes: 41977e86 ("rt2x00: add support for MT7620")
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>