ipmi: kcs: Poll OBF briefly to reduce OBE latency

The ASPEED KCS devices don't provide a BMC-side interrupt for the host
reading the output data register (ODR). The act of the host reading ODR
clears the output buffer full (OBF) flag in the status register (STR),
informing the BMC it can transmit a subsequent byte.

On the BMC side the KCS client must enable the OBE event *and* perform a
subsequent read of STR anyway to avoid races - the polling provides a
window for the host to read ODR if data was freshly written while
minimising BMC-side latency.

Fixes: 28651e6c ("ipmi: kcs_bmc: Allow clients to control KCS IRQ state")
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Message-Id: <20220812144741.240315-1-andrew@aj.id.au>
Signed-off-by: Corey Minyard <cminyard@mvista.com>