igc: Fix overwrites return value

drivers/net/ethernet/intel/igc/igc_i225.c:235 igc_write_nvm_srwr()
warn: loop overwrites return value 'ret_val'

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Dvora Fuxbrumer <dvorax.fuxbrumer@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>