mtd: physmap_of: fix cmdline partition method w/o linux, mtd-name

Commit d68cbdd4 (mtd: physmap_of: allow to specify the mtd name for retro
compatiblity) broke cmdline partitioning using dev_name() in the kernel command
line.  of_property_read_string() does not touch mtd_name when linux,mtd-name is
not present in the device tree, which causes map.name to be set to a random
value.  Fix this by initializing mtd_name to NULL.

Cc: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>