ixgbe: introduce a helper to simplify code

ixgbe_dbg_reg_ops_read and ixgbe_dbg_netdev_ops_read copy-pasting
the same code except for ixgbe_dbg_netdev_ops_buf/ixgbe_dbg_reg_ops_buf,
so introduce a helper ixgbe_dbg_common_ops_read to remove redundant code.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>