Missing nullptr check for API call in "net/phy.h"

[8t-tni]

Describe the bug

The function static inline int phy_link_callback_set(const struct device *dev, phy_callback_t callback,void *user_data) claims to return the error code "-ENOTSUP" if the API call is not supported.

But the function misses a correspondung nullptr check.

Current implementation:

static inline int phy_link_callback_set(const struct device *dev, phy_callback_t callback,
					void *user_data)
{
	const struct ethphy_driver_api *api = (const struct ethphy_driver_api *)dev->api;

	return api->link_cb_set(dev, callback, user_data);
}

Posseblie fix:

static inline int phy_link_callback_set(const struct device *dev, phy_callback_t callback,
					void *user_data)
{
	const struct ethphy_driver_api *api = (const struct ethphy_driver_api *)dev->api;
        if (api->link_cb_set != NULL)
	    return api->link_cb_set(dev, callback, user_data);
        else
            return -ENOTSUP;
}

To Reproduce
Steps to reproduce the behavior:

1. Create a little sample application using a STM32 nucleo H7
2. Try to set a callback using phy_link_callback_set
3. Run your sample application

Expected behavior
The function returns -ENOTSUP indicating that the ehternet driver does not support that API call.

Impact
The sample application crashes with a memory/access violation.

Logs and console output

Environment (please complete the following information):

• OS: Windows with VsCode and WSL/docker
• Toolchain Zephyr SDK

Additional context

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[kartben]

@8t-tni thanks for the report! Would you be interested in submitting a pull request to fix this?