i40e: remove redundant initialization of read_size

Variable read_size is initialized and this value is never read, it is
instead set inside the do-loop, hence the initialization is redundant
and can be removed. Cleans up clang warning:

drivers/net/ethernet/intel/i40e/i40e_nvm.c:390:6: warning: Value stored
to 'read_size' during its initialization is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>