drivers: flash: mark unused argument

Fixes:

```
.../zephyr/drivers/flash.h: In function 'flash_params_get_erase_cap':
.../flash.h:106:63: error: unused parameter 'p' [-Werror=unused-parameter]
  106 | int flash_params_get_erase_cap(const struct flash_parameters *p)
      |                                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
```

Signed-off-by: Nerijus Bendžiūnas <nerijus.bendziunas@astrolightspace.com>