zephyr: rename 'led_init()' to 'io_led_init()'

This fixes below warning when building with 'MCUBOOT_INDICATION_LED'
enabled:

  mcuboot/boot/zephyr/main.c:410:5:
    warning: implicit declaration of function 'led_init';
             did you mean 'io_led_init'? [-Wimplicit-function-declaration]
      410 |     led_init();
          |     ^~~~~~~~
          |     io_led_init

Fixes: 433b8480 ("zephyr: Move IO functions out of main to separate file")
Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>