serial: mux: Align SUPPORT_SYSRQ behaviour with other drivers.

The mux driver is anomalous among all the serial drivers that can
define SUPPORT_SYSRQ because it can, with some configs, set
SUPPORT_SYSRQ when SERIAL_CORE_CONSOLE is not set.

Not only does this impose a pointless (but tiny) runtime overhead for
such configs but, more significantly, it adds needless complexity when
doing a code review to check for unexpected side effects of any
changes to the serial core.

This is (cross-)compile tested only because I do not have any PA-RISC
hardware.

Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>