NVS: replace dev_name parameter by device reference in nvs_init()

[nvlsianpu]

Introduction

int nvs_init(struct nvs_fs *fs, const char *dev_name) might take device name which will help with cases when device_name is unknow to use.

Problem description

#33346 make flash_area API and flash_map operates with direct knowledge of flash device reference. Device name is still an option.
As NVS fetch device reference right in initialization, using device reference directly would be straight forward step.

Proposed change

change nvs API so nvs_init will API become:

int nvs_init(struct nvs_fs *fs, const struct device *dev)

[nvlsianpu]

closing in favor to #33627