gpiolib: cdev: Cleanup kfifo_out() error handling

gpiolib: cdev: Cleanup kfifo_out() error handling

The handling of kfifo_out() errors in read functions obscures any error.
The error condition should never occur but, while a ret is set to -EIO, it
is subsequently ignored and the read functions instead return the number
of bytes copied to that point, potentially masking the fact that any error
occurred.

Log a warning and return -EIO in the case of a kfifo_out() error to make
it clear something very odd is going on here.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Link: https://lore.kernel.org/r/20240529131953.195777-4-warthog618@gmail.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>