vme: tsi148: Improve 17 size determinations

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

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
[martyn@welchs.me.uk: Correct long line]
Signed-off-by: Martyn Welch <martyn@welchs.me.uk>