i2c: testunit: don't erase registers after STOP

i2c: testunit: don't erase registers after STOP

STOP fallsthrough to WRITE_REQUESTED but this became problematic when
clearing the testunit registers was added to the latter. Actually, there
is no reason to clear the testunit state after STOP. Doing it when a new
WRITE_REQUESTED arrives is enough. So, no need to fallthrough, at all.

Fixes: b39ab96aa894 ("i2c: testunit: add support for block process calls")
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Andi Shyti <andi.shyti@kernel.org>