mtd: spi-nor: Tidy up error handling / debug code

The -ENODEV error value in spi_nor_read_id() is incorrect since there
clearly is a device - it just cannot be supported. Use -ENOMEDIUM instead
which has the virtue of being less common.

Fix the return value in spi_nor_scan().

Also there are a few printf() statements which should be debug() since
they bloat the code with unused strings at present. Fix those while here.

Signed-off-by: Simon Glass <sjg@chromium.org>