serial/aspeed-vuart: Implement quick throttle mechanism

Although we populate the ->throttle and ->unthrottle UART operations,
these may not be called until the ldisc has had a chance to schedule and
check buffer space. This means that we may overflow the flip buffers
without ever hitting the ldisc's throttle threshold.

This change implements an interrupt-based throttle, where we check for
space in the flip buffers before reading characters from the UART's
FIFO. If there's no space available, we disable the RX interrupt and
schedule a timer to check for space later.

For this, we need an unlocked version of the set_throttle function to be
able to change throttle state from the irq_handler, which already holds
port->lock.

This prevents a case where we drop characters under heavy RX load.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Tested-by: Eddie James <eajames@linux.vnet.ibm.com>
Tested-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>