drivers: nrf_qspi_nor: Fix and refactor driver initialization

So far the driver first changed the configuration of the flash chip
and after that checked the signature of that chip. This could lead
to improper change of the chip configuration if the actually found
one was different than that specified in devicetree.
This commit reverses the order of these two initialization steps and
also restructures a bit the initialization code.

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