gpiolib: cdev: Refactor allocation of linereq events kfifo

The allocation of the linereq events kfifo is performed in two separate
places.  Add a helper function to remove the duplication.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Link: https://lore.kernel.org/r/20240529131953.195777-3-warthog618@gmail.com


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>