ar71xx: make bootconsole wait for both THRE and TEMT

Original commit message:

    MIPS: ath79: make bootconsole wait for both THRE and TEMT

    This makes the ath79 bootconsole behave the same way as the generic 8250
    bootconsole.

    Also waiting for TEMT (transmit buffer is empty) instead of just THRE
    (transmit buffer is not full) ensures that all characters have been
    transmitted before the real serial driver starts reconfiguring the serial
    controller (which would sometimes result in garbage being transmitted.)
    This change does not cause a visible performance loss.

    In addition, this seems to fix a hang observed in certain configurations on
    many AR7xxx/AR9xxx SoCs during autoconfig of the real serial driver.

    A more complete follow-up patch will disable 8250 autoconfig for ath79
    altogether (the serial controller is detected as a 16550A, which is not
    fully compatible with the ath79 serial, and the autoconfig may lead to
    undefined behavior on ath79.)