i2c: Improve size determinations

Replace the specification of a data structure by a pointer dereference
as the parameter for the operator "sizeof" to make the corresponding
size determination a bit safer according to the Linux coding style
convention.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Wolfram Sang <wsa@kernel.org>