i2c: testunit: avoid re-issued work after read message

The to-be-fixed commit rightfully prevented that the registers will be
cleared. However, the index must be cleared. Otherwise a read message
will re-issue the last work. Fix it and add a comment describing the
situation.

Fixes: c422b6a6 ("i2c: testunit: don't erase registers after STOP")
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Andi Shyti <andi.shyti@kernel.org>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>