serial: 8250_bcm7271: use default_mux_rate if possible

There is a scenario when resuming from some power saving states
with no_console_suspend where console output can be generated
before the 8250_bcm7271 driver gets the opportunity to restore
the baud_mux_clk frequency. Since the baud_mux_clk is at its
default frequency at this time the output can be garbled until
the driver gets the opportunity to resume.

Since this is only an issue with console use of the serial port
during that window and the console isn't likely to use baud
rates that require alternate baud_mux_clk frequencies, allow the
driver to select the default_mux_rate if it is accurate enough.

Fixes: 41a46948 ("serial: 8250: Add new 8250-core based Broadcom STB driver")
Cc: stable@vger.kernel.org
Signed-off-by: Doug Berger <opendmb@gmail.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Tested-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://lore.kernel.org/r/20240424222559.1844045-1-opendmb@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>