IB/hfi1: Coding style improvement (make sizeof use safer)

Replace the specification of a data structure by a reference to
the desired member 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>
Signed-off-by: Doug Ledford <dledford@redhat.com>