net: ethernet: et131x: Remove unused variable 'pm_csr'

Gcc report warning as follows:

drivers/net/ethernet/agere/et131x.c:953:6: warning:
 variable 'pm_csr' set but not used [-Wunused-but-set-variable]
  953 |  u32 pm_csr;
      |      ^~~~~~
drivers/net/ethernet/agere/et131x.c:1002:6:warning:
 variable 'pm_csr' set but not used [-Wunused-but-set-variable]
 1002 |  u32 pm_csr;
      |      ^~~~~~
drivers/net/ethernet/agere/et131x.c:3446:8: warning:
 variable 'pm_csr' set but not used [-Wunused-but-set-variable]
 3446 |    u32 pm_csr;
      |        ^~~~~~

After commit 38df6492

 ("et131x: Add PCIe gigabit ethernet driver
et131x to drivers/net"), 'pm_csr' is never used in these functions,
so removing it to avoid build warning.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>
Acked-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>