FRV: Switch unaligned access to the packed-struct implementation

Switch unaligned access to the packed-struct implementation for BE accesses as
this reduces the size of the kernel a little.  LE still uses the byte shift.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>