UBIFS: replace seq_printf by seq_puts

Fix checkpatch warnings:
"WARNING: Prefer seq_puts to seq_printf"

Andrew Morton wrote:

"
- puts is presumably faster

- puts doesn't go rogue if you accidentally pass it a "%".

- this patch actually made fs/ubifs/super.o 12 bytes smaller.
  Perhaps because seq_printf() is a varargs function, forcing the
  caller to pass args on the stack instead of in registers.
"

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>