drivers: nrf_qspi_nor: Clean up handling of return values

Consistently use `res` for results of calls to nrfx functions
and `rc` for Zephyr return codes, to avoid mixing up those two
and for example calling `qspi_get_zephyr_ret_code()` for a value
that is already a Zephyr return code. Correct also such call in
`qspi_nor_write()`.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>