atm: idt77252: remove unused function

Fix the following clang warning:

drivers/atm/idt77252.c:1787:1: warning: unused function
'idt77252_fbq_level' [-Wunused-function].

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>