dw2102: limit messages to buffer size

commit 950e252cb469f323740d78e4907843acef89eedb upstream.

Otherwise the i2c transfer functions can read or write beyond the end of
stack or heap buffers.

Signed-off-by: Alyssa Milburn <amilburn@zall.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
[bwh: Backported to 3.2:
 - Use obuf instead of state->data
 - Adjust filename, context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>