Out of bounds access in smc91c111_readb()
Description of problem:
I detected an out-of-bounds access in smc91c111_readb with my fuzzer.

Stack trace (part):\
`hw/net/smc91c111.c:607:24: runtime error: index 175 out of bounds for`\
`type 'uint8_t[4][2048]' (aka 'unsigned char[4][2048]')`\
`SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior`\
`hw/net/smc91c111.c:607:24 in`\
`AddressSanitizer:DEADLYSIGNAL`\
`==============================`<wbr>`==============================`<wbr>`=====`\
`==397944==ERROR: AddressSanitizer: SEGV on unknown address`\
`0x629000077db4 (pc 0x56272aed3b8d bp 0x7ffd1471f290 sp 0x7ffd1471ea20`\
`T0)`\
`==397944==The signal is caused by a READ memory access.`\
    `#0 0x56272aed3b8d in smc91c111_readb hw/net/smc91c111.c:607:24`\
    `#1 0x56272aecfd61 in smc91c111_readfn hw/net/smc91c111.c:650:16`\
    `#2 0x56272d4b228b in memory_region_read_accessor system/memory.c:445:11`\
    `#3 0x56272d46fb85 in access_with_adjusted_size system/memory.c:573:18`\
    `#4 0x56272d46c58e in memory_region_dispatch_read1 system/memory.c:1426:16`\
    `#5 0x56272d46bcd7 in memory_region_dispatch_read system/memory.c:1459:9`\
    `#6 0x56272d4e8e03 in flatview_read_continue_step system/physmem.c:2794:18`\
    `#7 0x56272d4e871e in flatview_read_continue system/physmem.c:2835:19`\
    `#8 0x56272d4e98b8 in flatview_read system/physmem.c:2865:12`\
    `#9 0x56272d4e9388 in address_space_read_full system/physmem.c:2878:18`\
    `#10 0x56272d6e7840 in address_space_read include/exec/memory.h:3026:18`\
`...`\
Bug analysis: I found s-\>packet_num = 175 at line 599.
Steps to reproduce:
Reproducer:\
export QEMU_ARGS="-display none -machine accel=qtest, -m 512M -machine\
mainstone"\
cat \<\< EOF | ./qemu-system-arm $QEMU_ARGS -qtest /dev/null -qtest stdio\
outl 0xcf8 0x80000010\
outl 0xcfc 0x10000300\
outl 0xcf8 0x80000004\
outl 0xcfc 0x07\
writel 0x1000030c 0x66027cd6\
writel 0x10000300 0x64af8eda\
readw 0x10000308\
EOF
Additional information:
Ack: Chuhong Yuan (hslester96@gmail.com)