ixgbe: fix signed-integer-overflow warning

commit 3b70683f upstream.

ubsan report this warning, fix it by adding a unsigned suffix.

UBSAN: signed-integer-overflow in
drivers/net/ethernet/intel/ixgbe/ixgbe_common.c:2246:26
65535 * 65537 cannot be represented in type 'int'
CPU: 21 PID: 7 Comm: kworker/u256:0 Not tainted 5.7.0-rc3-debug+ #39
Hardware name: Huawei TaiShan 2280 V2/BC82AMDC, BIOS 2280-V2 03/27/2020
Workqueue: ixgbe ixgbe_service_task [ixgbe]
Call trace:
 dump_backtrace+0x0/0x3f0
 show_stack+0x28/0x38
 dump_stack+0x154/0x1e4
 ubsan_epilogue+0x18/0x60
 handle_overflow+0xf8/0x148
 __ubsan_handle_mul_overflow+0x34/0x48
 ixgbe_fc_enable_generic+0x4d0/0x590 [ixgbe]
 ixgbe_service_task+0xc20/0x1f78 [ixgbe]
 process_one_work+0x8f0/0xf18
 worker_thread+0x430/0x6d0
 kthread+0x218/0x238
 ret_from_fork+0x10/0x18

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Xie XiuQi <xiexiuqi@huawei.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>