regmap: fix NULL pointer dereference in _regmap_write/read

If LOG_DEVICE is defined and map->dev is NULL it will lead to NULL
pointer dereference. This patch fixes this issue by adding check for
dev->NULL in all such places in regmap.c

Signed-off-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org