backlight/gpio_backlight: Rename field 'fbdev' to 'dev'

Rename the field 'fbdev' in struct gpio_backlight_platform_data and
struct gpio_backlight to 'dev', as they store pointers to the Linux
platform device; not the fbdev device. Makes the code easier to
understand.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Rich Felker <dalias@libc.org>
Cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Cc: Lee Jones <lee@kernel.org>
Cc: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Jingoo Han <jingoohan1@gmail.com>
Cc: linux-sh@vger.kernel.org
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230613110953.24176-5-tzimmermann@suse.de