i2c: rcar: use flags instead of atomic_xfer

i2c-rcar already has priv->flags. This patch adds a new persistent flag
ID_P_NOT_ATOMIC and uses it to save the extra variable. The negation of
the logic was done to make the code more readable.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
[wsa: negated the logic, rebased, updated the commit message]
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>