Change rgblight_get_mode & rgb_matrix_get_mode's return type to uint8_t. (#4747)

Change rgblight_get_mode & rgb_matrix_get_mode's return type to uint8_t. (#4747)

* Change rgblight_get_mode's return type to uint8_t.

Since rgblight_get_mode() is just returning rgblight_config_t.mode,
it should match rgblight_config_t.mode's type: uint8_t.

* Update rgb_matrix_get_mode to return uint8_t.