staging: unisys: remove OFFSETOF from commontypes

The only place OFFSETOF was being used was in iochannel.h. Remove the macro
from commontypes.h and replace all uses of it in iochannel.h with offsetof()
instead.

Signed-off-by: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>