fbdev: ssd1307fb: Print the PWM's label instead of its number

struct pwm_device::pwm is a write-only variable in the pwm core and used
nowhere apart from this and another dev_dbg. So it isn't useful to
identify the used PWM. Emit the PWM's label instead in the debug
message.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Helge Deller <deller@gmx.de>