drivers: SPI NOR: Inconsistency with flash interface docs

[gon1332]

Describe the bug
The addition of SPI NOR flash driver introduced an incosistency with the docs for flash.
According to the docs, flash_read/flash_write return 0 on success. On the contrary, spi_nor_read/spi_nor_write return the amount of bytes read/written from/to spi flash.

The big difference can be seen in the usage of flash interface in spi_flash sample vs this spi_flash test in which the reader of the code should assume the underlying hardware flash scheme. This destroys the abstraction that should be taken for granted.

Impact
I created a sample for my board that has a SPI NOR. It took me some time to see why the tests were failing, since I followed the docs descriptions.

[gon1332]

This issue is fixed by #11628.