i2c: imx-lpi2c: clean rx/tx buffers upon new message

[ Upstream commit 987dd36c ]

When start sending a new message clear the Rx & Tx buffer pointers in
order to avoid using stale pointers.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Tested-by: Emanuele Ghidoli <emanuele.ghidoli@toradex.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>