mtd: sm_ftl: fix typo in major number.

commit 452380ef upstream.

major == 0 allocates dynamic major, not major == -1

Signed-off-by: Maxim Levitsky <maximlevitsky@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>