i2c: testunit: discard write requests while old command is running

When clearing registers on new write requests was added, the protection
for currently running commands was missed leading to concurrent access
to the testunit registers. Check the flag beforehand.

Fixes: b39ab96a ("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>