drivers: flash: spi_nor: fix flash busy during init

After a soft-reset of the host controller the flash device is not reset.
This can cause the flash device to still be busy writing or erasing when
the host controller boots and initializes the flash. The often results
in errors and the flash device not being initialized.

This fix polls the status register until the WIP flag is off before
initializing the flash device.

Fixes #51713

Signed-off-by: Gregers Gram Rygg <gregers.gram.rygg@nordicsemi.no>