tty: serdev: core: Fix misspelled function name __serdev_device_driver_register()

tty: serdev: core: Fix misspelled function name __serdev_device_driver_register()

Fixes the following W=1 kernel build warning(s):

 drivers/tty/serdev/core.c:809: warning: expecting prototype for serdev_driver_register(). Prototype was for __serdev_device_driver_register() instead

Cc: Rob Herring <robh@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: linux-serial@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20210520121906.3468725-3-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>