mchp/espi: fix error condition handling
mchp/espi: fix error condition handling

As uint tidx can't be < 0 which can lead to overflow further down.

(Found by Coverity Scan)

BUG=none
BRANCH=none
TEST=none

Change-Id: I63988be98a64292362cdc017beceac296ddde0dc
Signed-off-by: Patrick Georgi <bc411205f21846a74924ee7b489c75617ec76078@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/1768650
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