mtd: nettel: do not ignore mtd_device_register() failure in nettel_init()

If mtd_device_register() fails in nettel_init(), iomap left mapped.

The patch adds failure handling for mtd_device_register().

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>