e1000e: remove the redundant value assignment in e1000_update_nvm_checksum_spt

Both of the statements are value assignment of the variable act_offset.
The first value assignment is overwritten by the second and is useless.
Remove it.

Reported-by: Tosk Robot <tencent_os_robot@tencent.com>
Signed-off-by: Kaixu Xia <kaixuxia@tencent.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>