common: avb_verify: Fix division by zero in mmc_byte_io()

Compiling U-Boot with ubsan/asan libraries and running it in sandbox
may lead to below backtrace:

 => avb init 0
 => avb verify
 ## Android Verified Boot 2.0 version 1.1.0
read_is_device_unlocked not supported yet
common/avb_verify.c:407:31: runtime error: division by zero
AddressSanitizer:DEADLYSIGNAL
Reviewed-by: Igor Opaniuk <igor.opaniuk@linaro.org>

=================================================================
==9388==ERROR: AddressSanitizer: FPE on unknown address 0x0000004b467f \
    (pc 0x0000004b467f bp 0x000000000000 sp 0x7ffd899fe150 T0)
    #0 0x4b467e in mmc_byte_io common/avb_verify.c:407
    #1 0x4b4c47 in mmc_byte_io common/avb_verify.c:532
    #2 0x4b4c47 in read_from_partition common/avb_verify.c:533
    #3 0x69dc0d in load_and_verify_vbmeta lib/libavb/avb_slot_verify.c:560
    #4 0x6a1ee6 in avb_slot_verify lib/libavb/avb_slot_verify.c:1139
    #5 0x45dabd in do_avb_verify_part cmd/avb.c:245
    #6 0x4af77c in cmd_call common/command.c:499
    #7 0x4af77c in cmd_process common/command.c:538
    #8 0x46bafc in run_pipe_real common/cli_hush.c:1677
    #9 0x46bafc in run_list_real common/cli_hush.c:1875
    #10 0x46c780 in run_list common/cli_hush.c:2024
    #11 0x46c780 in parse_stream_outer common/cli_hush.c:3216
    #12 0x46d34b in parse_file_outer common/cli_hush.c:3299
    #13 0x4ad609 in cli_loop common/cli.c:217
    #14 0x4625ae in main_loop common/main.c:65
    #15 0x46f2d1 in run_main_loop common/board_r.c:648
    #16 0x640253 in initcall_run_list lib/initcall.c:30
    #17 0x46f9d0 in board_init_r common/board_r.c:879
    #18 0x40539b in main arch/sandbox/cpu/start.c:321
    #19 0x7fa94925f82f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
    #20 0x408908 in _start (/srv/R/u-boot-master/u-boot+0x408908)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: FPE common/avb_verify.c:407 in mmc_byte_io
==9388==ABORTING

Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>