ubifs: Fix type of sup->hash_algo

The sup->hash_algo is a __le16, and whilst 0xffff is
the same in __le16 and u16, it would be better to use
cpu_to_le16() anyway (which should deal with constants)
and silence the following sparse warning:

fs/ubifs/sb.c:187:32: warning: incorrect type in assignment (different base types)
fs/ubifs/sb.c:187:32:    expected restricted __le16 [usertype] hash_algo
fs/ubifs/sb.c:187:32:    got int

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>