e1000: drop unneeded assignment in e1000_set_itr()

The variable 'current_itr' is assigned to 0 before jumping to
'set_itr_now' but it has not been used after the jump. So, remove the
unneeded assignment.

Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Reviewed-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>