mtd: simplify get_mtd_device_nm function

'get_mtd_device_nm()' has a piece of code which equivalent to what
'__get_mtd_device()' does - remove this duplicated code and use
''__get_mtd_device()' instead.

Artem: changed commit message.
Artem: while on it, remove an unnecessary extra empty line

Signed-off-by: Wanlong Gao <wanlong.gao@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>