nanddump: check write function result for errors

nanddump: check write function result for errors

Errors may happen, it's e.g. easy on embedded devices to run out of space
when dumping big partitions. This patch adds a helper function for
writing. It deals with partial writes and just returns 0 on success or
error number.

The old code didn't check for errors at all which could result in
incomplete dumps without exiting with an error.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>