drivers: media: imx708: Tidy-ups to address upstream review comments

This commit addresses vaious tidy-ups requesed for upstreaming the
IMX708 driver. Notably:

- Remove #define IMX708_NUM_SUPPLIES and use ARRAY_SIZE() directly
- Use dev_err_probe where possible in imx708_probe()
- Fix error handling paths in imx708_probe()

Signed-off-by: Naushir Patuck <naush@raspberrypi.com>