[POWERPC] VIOTAPE: Use designated initializers for fops member structures.

Replace the old-style member initializers with the newer designated
initializers.

Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>