net: e1000: Remove set but not used variable

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/ethernet/intel/e1000/e1000_hw.c: In function e1000_phy_init_script:
drivers/net/ethernet/intel/e1000/e1000_hw.c:132:6: warning: variable ‘ret_val’ set but not used [-Wunused-but-set-variable]

`ret_val` is never used, so remove it.

Signed-off-by: Zheng Yongjun <zhengyongjun3@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>