[MTD] Auto-load mtdchar module when device opened.

The mtdchar module is missing the char-major-90-* alias that would cause
it to be auto-loaded when a device of that type is opened.  This patch
adds the alia..

Signed-off-by: Scott James Remnant <scott@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>