net: cxgb3: add error handling for sysfs_create_group

When sysfs_create_group fails, the lack of error-handling code may
cause unexpected results.

This patch adds error-handling code after calling sysfs_create_group.

Signed-off-by: Zhouyang Jia <jiazhouyang09@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>