hw/char/serial: Clean up unnecessary code

Since commit 5ec3a23e

 ("serial: convert PIO to new memory
api read/write") we don't need to worry about accesses bigger
than 8-bit. Use the extract()/deposit() functions to access
the correct part of the 16-bit 'divider' register.

Reported-by: Jonathan D. Belanger <jbelanger1@rochester.rr.com>
Buglink: https://bugs.launchpad.net/qemu/+bug/1904331


Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20201120161933.2514089-1-f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>