The ISA serial port handling in serial-isa.c imposes a limit
of 4 serial ports. This is because we only know of 4 IO port
and IRQ settings for them, and is unrelated to the generic
MAX_SERIAL_PORTS limit, though they happen to both be set at
4 currently.

Use a new MAX_ISA_SERIAL_PORTS wherever that is the correct
limit to be checking against.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Tested-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180420145249.32435-11-peter.maydell@linaro.org