[IA64] Remove sparse warning from unwind code

Running ia64 through sparse gives warnings in the unwind code.

  include/asm-ia64/unwind.h:84:17: error: dubious bitfield without explicit `signed' or `unsigned'

Make the bitfield explicitly unsigned.

Signed-off-by: Keith Owens <kaos@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>