hw/char/pl011: Avoid division-by-zero in pl011_get_baudrate()

hw/char/pl011: Avoid division-by-zero in pl011_get_baudrate()

In pl011_get_baudrate(), when we calculate the baudrate we can
accidentally divide by zero. This happens because although (as the
specification requires) we treat UARTIBRD = 0 as invalid, we aren't
correctly limiting UARTIBRD and UARTFBRD values to the 16-bit and 6-bit
ranges the hardware allows, and so some non-zero values of UARTIBRD can
result in a zero divisor.

Enforce the correct register field widths on guest writes and on inbound
migration to avoid the division by zero.

ASAN log:
==2973125==ERROR: AddressSanitizer: FPE on unknown address 0x55f72629b348
(pc 0x55f72629b348 bp 0x7fffa24d0e00 sp 0x7fffa24d0d60 T0)
     #0 0x55f72629b348 in pl011_get_baudrate hw/char/pl011.c:255:17
     #1 0x55f726298d94 in pl011_trace_baudrate_change hw/char/pl011.c:260:33
     #2 0x55f726296fc8 in pl011_write hw/char/pl011.c:378:9

Reproducer:
cat << EOF | qemu-system-aarch64 -display \
none -machine accel=qtest, -m 512M -machine realview-pb-a8 -qtest stdio
writeq 0x1000b024 0xf8000000
EOF

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Zheyu Ma <zheyuma97@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20240702155752.3022007-1-zheyuma97@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>