i2c: prevent endless uevent loop with CONFIG_I2C_DEBUG_CORE

i2c: prevent endless uevent loop with CONFIG_I2C_DEBUG_CORE

Jan reported this:

===
After enabling CONFIG_I2C_DEBUG_CORE my system was broken
(no network, console login not possible). System log was
flooded with the this message:

 ...
[  608.052077] rtc-ds1307 0-0068: uevent
[  608.052500] rtc-ds1307 0-0068: uevent
[  608.052925] rtc-ds1307 0-0068: uevent
 ...

The culprit is the dev_dbg printk in the i2c uevent handler.
If this is activated (for instance by CONFIG_I2C_DEBUG_CORE)
it results in an endless loop with systemd-journald.

This happens if user-space scans the system log and reads the uevent
file to get information about a newly created device, which seems fair
use to me. Unfortunately reading the "uevent" file uses the same
function that runs for creating the uevent for a new device,
generating the next syslog entry.

Ideally user-space would implement a recursion detection and
after reading the same device file for the 1000th time call it a
day, but nevertheless I think we should avoid this problem by
removing the debug print completely or using another print variant.

The same problem seems to be reported here:
https://bugs.freedesktop.org/show_bug.cgi?id=76886
===

His patch converted the message to pr_debug, but I think the debug can
simply go. We have other means to see code paths these days. This
enables us to clean up the function some more while we are here.

Reported-by: Jan Glauber <jglauber@cavium.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Acked-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Tested-by: Jan Glauber <jglauber@cavium.com>