device: remove 'extern' attribute from function prototypes in device.h

Functions are declared 'extern' implicitly by the compiler. There's no
reason to prepend every prototype with it. Remove the 'extern' keyword
from all function declarations in linux/device.h.
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Link: https://lore.kernel.org/r/20200629065008.27620-4-brgl@bgdev.plSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>