i2c: xiic: Fix broken locking on tx_msg

[ Upstream commit c119e7d0 ]

The tx_msg is set from multiple places, sometimes without locking,
which fall apart on any SMP system. Only ever access tx_msg inside
the driver mutex.

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>
Stable-dep-of: 1d4a1adb ("i2c: xiic: Try re-initialization on bus busy timeout")
Signed-off-by: Sasha Levin <sashal@kernel.org>