cxgb3: Fix __must_check warning with dev_dbg.

Fix the warning:
drivers/net/cxgb3/cxgb3_main.c: In function ‘offload_open’:
drivers/net/cxgb3/cxgb3_main.c:936: warning: ignoring return value of
 ‘sysfs_create_group’, declared with attribute warn_unused_result

Now the return value is checked; if sysfs_create_group() returns failure,
a warning is printed using dev_dbg, and the code continues as before.  Use
of dev_dbg ensures printk is not needlessly included unless desired for
debugging.

Signed-off-by: Dan Noe <dpn@isomerica.net>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>