i40e: remove BUG_ON from feature string building

There's really no reason to kill the kernel thread just because of a
little info string. This reworks the code to use snprintf's limiting to
assure that the string is never too long, and WARN_ON to still put out
a warning that we might want to look at the feature list length.

Prompted by a recent Linus diatribe.

Change-ID: If52ba5ca1c2344d8bf454a31bbb805eb5d2c5802
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>