hw/char/serial: Assert serial_ioport_read/write offset fits 8 bytes

hw/char/serial: Assert serial_ioport_read/write offset fits 8 bytes

The serial device has 8 registers, each 8-bit. The MemoryRegionOps
'serial_io_ops' is initialized with max_access_size=1, and all
memory_region_init_io() callers correctly set the region size to
8 bytes:
- serial_io_realize
- serial_isa_realizefn
- serial_pci_realize
- multi_serial_pci_realize

It is safe to assert the offset argument of serial_ioport_read()
and serial_ioport_write() is always less than 8.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200907015535.827885-2-f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>