hw/char/pl011: ensure UARTIBRD register is 16-bit

The PL011 TRM says that "The 16-bit integer is written to the Integer Baud Rate
Register, UARTIBRD". Updated the handling of the UARTIBRD register to ensure
only 16-bit values are written to it.

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

Signed-off-by: Zheyu Ma <[email protected]>
Message-Id: <[email protected]>

hw/char/pl011.c

@@ -374,7 +374,7 @@ static void pl011_write(void *opaque, hwaddr offset,
         s->ilpr = value;
         break;
     case 9: /* UARTIBRD */
-        s->ibrd = value;
+        s->ibrd = value & 0xFFFF;
         pl011_trace_baudrate_change(s);
         break;
     case 10: /* UARTFBRD */