common/flash.c: Support finding the flash_bank_index for offset 0
common/flash.c: Support finding the flash_bank_index for offset 0

According to the algorithm that's always the first entry, but it always
returned -1 so far.

Found by Coverity Scan #174434

BUG=none
BRANCH=none
TEST=none

Change-Id: Idd3f2794b559bea47f8425791351a8d0bc1424d6
Signed-off-by: Patrick Georgi <bc411205f21846a74924ee7b489c75617ec76078@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/1793583
Reviewed-by: Daisuke Nojiri <fd5f93af191bf7e8f73ea71cc3c0f66b41b1dd49@chromium.org>
Tested-by: Patrick Georgi <bc411205f21846a74924ee7b489c75617ec76078@chromium.org>
Commit-Queue: Patrick Georgi <bc411205f21846a74924ee7b489c75617ec76078@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec